RECORD: Darwin, C. R. Geological diary: Uspallata Pass. [4.1835] CUL-DAR36.502-549 Transcribed by Kees Rookmaaker, edited by John van Wyhe. (Darwin Online, http://darwin-online.org.uk/)
REVISION HISTORY: Transcribed by Kees Rookmaaker, corrected and edited by John van Wyhe, corrections by Gordon Chancellor 11.2011. RN3
NOTE: This document, part of the largest scientific document composed by Darwin during the voyage of the Beagle, is written mostly in ink. Marginal notes are here integrated into the text. See the Beagle diary p. 550A.
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Reproduced with permission of the Syndics of Cambridge University Library and William Huxley Darwin.
1835 Mendoza. — St Jago: Uspallata Pass — 37
My return to St Jago was by the Uspallata Pass. As in the Portillo, the road crosses two ridges; the East one of Uspallata is of such low comparative altitude, that it is hardly ever considered. Leaving the Pampas we
continued began our ascent by the ravine of Villa Vicencia. — All the first low oblique ridges (or rather from degradation they deserve the name of wedges) consist of dusky green greywacke: This is sometimes coarse, & alternates with a commoner variety, passing into clay slate. —
Judging from fragments on the plains, this formation is extensive, extending on the extreme East slope of mountains at least as far South as Mendoza. — I believe at La Calera, the grey crystalline limestone belongs to this formation. — In the mountains in the vicinity of the house of Villa Vicencia (a), a
true blue much laminated micaceous clay slate is most abundant 2045, is associated with some beds of a coarse greywacke containing bits of quartz & slate. 2647 — The laminae of slate are traversed by numerous transverse veins of quartz. — The whole formation strictly resembled the common strata of the outskirts of North Wales. — The clay slate & greywacke
(a) V. Vicencia according to Mr Miers.1 has an elevation of 5382 ft above the sea & 2780 above Mendoza. — The surrounding mountains must reach elevation of at least 7000 ft. —
These several hot springs varying (Miers) from 96° to 75°. — Their taste is slightly saline.
Mr Miers states that quantities of
Carb Gas are is disengaged from the bottom I was unable did not observe this ——
1 Miers 1826.
1835 Uspallata Pass 38
alternate & have a fissile character, in planes, which in the outer parts are irregular in strike but nearly vertical in angle. — Perhaps
N NE & SW is the common line: — more within the mountains, the strike is about N & S & the dip universally to the W at angle from 70° 45° to 80°. —
(a) contorted distorted clear
Close to the spot, where the hot springs are situated there is an appearance of an anticlinal line. — Certainly to the West of it we meet with the westerly dip. — From what I have seen in the Falkland Isd & T. del Fuego, I must remain in doubt concerning the nature of the laminated & changing in mechanical structure, whether they are owing to true stratification or cleavage. —
Following up the ravine of the springs & ascending some high hills, I found the blueish clay slate in all parts covered by a purple variety; this again was succeeded by a varying mass of strata. — The lower bed of no great thickness is a coarse white conglomerate, cement partly calcareous containing grains of quartz & feldspar & a few rounded pebbles of porphyry. 2649 —
The commonest variety of this latter rock is very compact & purple latter, containing crystals of red feldspar. 2650 — This is followed by great
(a) The strata here have been contorted & dislocated
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mass of a fine grained sandstone 2651, particles united by a white cement, & marked with ferruginous water lines.
sandstone greystone pebbles
There is some pale purple yes sandstone 2652, grain not siliceous, which in parts passes into a very compact substance.
like Pater bed [baked valuted] —
Beds of a white,
[illeg] hard, earthy fracture slightly calcareous substance, like decomposed feldspar 2653, containing ferruginous matter in patches, is very abundant. — This is associated with harder more compact kinds, sometimes containing extraneous particles 2654, & of a faint green color. —
These pass into a blueish & greenish rock, which are closely connected
to othe in nature with clay. slate. — Also above the coarse conglomerate I met in one spot some true but compact blue clay. slate. — In this section the beds are confused & pass into each other, in another they are clearly stratified. — In the higher land to the West it is seen to be an enormous formation: I picked up numerous fragments of crystalline rock. — All the strata dip to W at angle from 20°-30° or even less. — On a grand scale, in the landscape
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as well as in the individual observations, it was evident that the laminae of slate dipped to the West at a higher angle than the superincumbent strata. Yet we have seen that the clay slate changes color near junctions; that substances, closely allied to it, are produced above the junction, & lastly in the conglomerate there are no fragments of slate. — In some spots the cleavage of slate
became were less inclined, high up, & the whole appeared conformable. —
But otherwise, I saw at the base of the hill the nearly vertical slate & above the white beds & sandstone. — I took much trouble to find a good section, but did not succeed of coming within some feet of the junction. — In ascending thinly laminated blue slate, became rotten, the cleavage being slightly curvilinear & crossed by another system of planes of division. Hence the whole was a mass of splinters; here the color had changed into a brown & greenish tinge & the vertical cleavage scarcely perceptible, this was succeeded by a purple, slate, the angle of
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inclination of which was small & conformable to the superior beds. — As this is a point of some consequence I will describe
part of a section in the common road. from V. Vicencia to los Hornillos. — we first had the clay slate & greywacke dipping at ∠80° to S & SSW. & as the first of the other strata dip 30° to W. nothing can appear more unconformable yet. near to the junction the slate obtains an irregular cleavage (a) & is associated with a red greywacke, & in another section some blue & red slates appear quite conformable to the whole strata. — I am strongly urged to believe that the apparent stratification of the slate, has no relation as in the Falkland Isd with the plane of deposition & that the superincumbent strata were accumulated on the slate, before its upper strata were consolidated, the change being attributed to a lapse of time or alteration of circumstances. — On another point of view they must be considered as a line of irregular hills preexisting at bottom of ocean during the age of subsequent deposition & the rehardened mud from their degradation, produced the appearance of a passage. —
(a) Here There is a broard dyke of a white soft
rock substance 2694 2694, which certainly strongly resembles some of the neighbouring sedimentary beds & not a volcanic rock — The dyke is nearly vertical & broard by its irregular outline I know it is not a bed in the slate, but I am unable to say whether it is a fissure filled up from above or below. if the latter, the preexistence of the slate is proved. — Perhaps an examination of the specimen will be conclusive. — The metamorphic action in the sedimentary beds will render this difficult. —
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This section of the road lies to the North of the ravine of the springs: accordingly we meet the white sedimentary beds in a line due North of the mountain there described. — These strata are much the same, & several hundred feet thick. — They are covered by a bed, about 100 ft thick if white feldpathic rock with crystals of do
2692 2693; the structure is slightly partially curved columnar: when it rests on the sedimentary strata (yes), they are hardened with a semi sphaerico-concretionary structure in this spot the color is purplish owing apparently to their being many large pebbles of fine purple compact porphyry containing grains of quartz. —
have should suppose such pebbles have originally come from the main Cordilleras. —
This crystalline bed is covered by about 400 ft of thinly stratified sedimentary of varying nature, which all dip from 25°. 30° to the W. — Numerous little pebbles of quartz
2624 2639 are cemented together by a white ferrugino-aluminous base, with little concretionary balls of a white substance, like decomposed feldspar: such substances in some cases are finer grained 2690, & again there are light brown compact sandstones 2691, where the particles are not siliceous. — There is also a good deal
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of a very compact, aluminous stone of a blueish or green color like (2684: 85). — On some strata such as these an extraordinary bed of crystalline rocks lies: its thickness may be 250 ft. — I do not believe a hundred specimens would show all the kinds 2673 ... 2682. a good deal is amygdaloid — The only thing I have seen at all like the variations, which appear to obey no law
was is at S. Carlos de Chiloe. — At a distance the inferior straight line of separation is very manifest: on close inspection, however the two substances for the thickness of a foot seem quite blended & the united substance is very curious. 2683 — The section so far described is close to & below the house of Trapiche of Los Hornillos. — What follows is just behind it. —
2660 Black Lime
2655 Feldspathic base
Feldspathic Lava 2658
x curious porphyries
8 7 6 5 4 3 2 1
Hill about 450 ft
Above the stratum of curious porphyries & crystalline rocks we have about 80 ft, of such sedimentary beds, as have been described, namely a more or less hard, white decomposed feldspathic base, which is sometimes siliceous & hard, & contains balls of chlorite, or merely extraneous particles. 2656 2655 — The base is oftentime granular crystalline. These white aluminous ferruginous kinds of sandstones, are very peculiar. —
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There is also much blueish grey compact aluminous stone, often possessing a porcellanous structure 2684 2695; this is here remarkable by vegetable impressions. —
I do not mean here to discuss age of these formations, but the resemblance to parts of Chiloe, Patagonia, Concepcion was very striking; these being rather harder. —
Bed 2d. Above this comes a bed
th 50 ft thick of a grey cryst feldspathic rock with little angite (?) in interstices 2058; the inferior junction is not well defined; the altered strata containing irregular balls & masses of the crystalline rock. the appearance is not however by any means that of an injected rock. #
Bed 3d. This bed is again capped by the same class of sedimentary rocks. — which contains a very remarkable bed of pitchstone, this generally has a brecciated structure
B: 4th. 2659. the stratum is regular & conformable & 2 ft thick. I cannot suppose this a melted rock, but rather some bituminous clay altered; the manner in which the little pieces
fall are arranged in lines resembles what I have seen in the lignites of Concepcion. —
B. 5th. In the sandstone immediately above the pitchstone, there is some conglomerate where some of the well-rounded pebbles of porphyries are larger than
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a cricket ball. — The sandstone is marked with ferruginous water lines. —
B: 6th. Again we have a bed of a feldspathic rock, sonorous, with large angular planes of divisions, as before,
B: 7th. covered by more sandstones & there capped
B: 8th. by a thick bed of a black augitic? rock; the inferior junction is here tolerably well defined 2660 2661, each class being perfect within 2 inches of the junction. — I believe to the West there were many more similar alternations, but I had not time to examine them. —
All the above strata dip to the West, at about 25°; but on the top of the hill the dip is directly contrary to the East at about same angle. — This synclinal line seems to separate two whole districts with opposite dips. yet it is remarkable that it occurs in the slope of a hill & very little affect the external outline.
The part to the West of the above the section, the strata of which dip Easterly will be presently described. — It appears to me certain, that the above five
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grand beds of crystalline rocks, alternating with those singular sedimentary ones, have been subaqueous lavas. — It must be remembered, that the waters were charged with carb of lime, silex & I believe gypsum; it will be shown, that metallic veins have affected the rocks to considerable distances & that the whole probably has undergone the action of heat. — All the above strata, both crystall & the sedimentary have been traversed by exceedingly numerous thin metallic veins; these are generally nearly vertical, but interlaces so as to form network. —
The basis of the veins is iron, which contains either a little gold or silver in disseminated particles. Specimen (2686) 2686 is a poor one of the latter.
The effect of these veins on the adjoining strata is very remarkable, whole masses being blackened. hardened & rendered heavier;
This is most clearly seen in the white sedimentary rocks. 2687 2688 2692. — The appearance is that of a shattered rock all the adjoining fissures burnt & blackened by metallic fumes. —
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The veins in the immediate neighbourhead of the Hornillos have not been worked, but a few miles to the West, in central parts of the Uspallata range there are several gold mines. —
I rode to these. — After passing the section already described, we entered on the same class of strata, all dipping from 25° to 30° to the East. — I observed in many plains the sedimentary white rocks very much altered by the metallic veins. — The summit of the Uspallata range is broard & uneven, the strata within the Eastern ridge are likewise uneven. — I passed on to an irregular mass of granite hills; at some distance to the NNW there was another & still loftier hill of this rock. — All these must attain an elevation of little less than 8000 ft above the sea. — On the
West East side all the strata have a westerly dip; so that the broard central axis of granite forms a very regular anticlinal line in the stratification. —
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The granite, has all
the its usual characters, but is composed of white feldspar, 2663 black mica & little hornblende, but none no or but very little quartz; there are veins of epidote. — It is of the class of grey granites & syenitic greenstone of the Portillo pass. — Round the base of these hills, there is a casing of the stratified formation. the rocks are however much altered being harder & more crystalline. 2662. —
One of the highest peaks was capped by a considerable mass of such strata; their character so altered I could only just recognize them. — The flinty & jaspery strata are interlaced with small granitic veins. — The feldspar is there in smally crystallized 2664 & contain which is singular rounded grains of quartz. — Specimen shows junction of one these veins & a blackish siliceous rock. —
Seeing the intense igneous action here & observing appearances which might be accounted for by same cause in the whole anticlinal band. I have no doubt, the inferior once melted granite has thus affected them all. —
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The metallic veins occur both in the granite strata & in the strata which
sup overlie & case it; They are excessively numerous & here all auriferous. — The direction of the principal ones is NW & SE. —
The first mines I examined were in the granite, a little way below the Cape, above mentioned. — The veins "vetas" were parallel & about a yard wide, composed of imperfectly crystall: ferruginous feldspar 2665. — in these were the auriferous, ferruginous "gias" of which specimens 2666 is a good one & will show the width. —
The mine is worked at "Crucero" or point of intersection of the "gias". — The Bogue mine is perhaps the best, it is situated a few hundred feet lower down & in the curious & altered sedimentary rock, which unquestionably only case the granite.
The strata here are composed of a compact. white. conch: fract. siliceo. feldspathic rock. 2667 2668 & one of the white aluminous kind 2669 ... 2672; (2669) is one of the best: (2670: 71) the common sorts, one of which contains some copper: (2672) is in the state of clay. lower down contains much iron pyrites, is expected to turn out well. —
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The general resemblance, of the ores & accompanying minerals. — the feldspathic veta. — &c then to the mines
of Yaquil in the granitic hills of Yaquil is very striking. — At no great distance, in the same formations silver mines have been attempted & I saw some most excellent copper pyrites. 2712 —
It is very clear, that all the metallic veins proceed
from through the inferior granite, which has manifestly tilted & altered the superincumbent strata. —
In one spot I actually traced a vein from one to the other formation. —
The section by the common road lies to the North of the one to the above mines. — Untill we reach the westerly dip on the West side the strata of sedimentary & volcanic rocks are undulating. We then descend very gradually, the stratification on each hand being inclined at a general angle about 25° to within a point of West. — The angles however owing to faults vary. [sketch] I saw some at about 10° & others at 45°. —
The volcanic rocks on this West side are much more abundant, than on the other; this is what would be expected, if their source is in the Cordilleras. 2696 common variety. — They are commonly of a black color
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are more apparently lava. possess a sphaerico & even columnar structure. — the superior junction of the sedimentary strata is very well defined. —
The sandstone are such as have been described variously sized small particles imbedded in the white aluminous base. — They are traversed by veins of cryst carb of line & nodules of agate. 2697 — In much of these beds there is a concretionary structure, I noticed in one spot these forms determined by lines of fissure. precisely, only on a larger scale, as figured in Chiloe at P. Huapacho. — In two places there were considerable quantities of thin layers of
less pitchstone rather less glistening than before; & a good deal of rounded little bits arranged in layers & imbedded in hard rock 2695.
I also saw one angular isolated fragment in the sandstone, whatever may be the origin of this substance such thin layers could never have flowed as a melted rock. — Further to the West & [were] I think there has been less metamorphic action: these were curved laminated, very compact siliceous jet black clay slates inducted shelf: on wetting one of the surfaces, vegetable impressions can be distinguished.
The extensive undulating layers appear to follow some
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irregularities in the inferior lava. — Low down on the West slope the strange variety of rocks quite defies my short descriptions. I can only mention some of the most particular. —
There was some of a quite white soft aluminous stone with few particles of quartz such as in Chiloe: again a similar stone 2718 only rather harder & more compact & much of those bluish & greenish porcellanous stones, which before had vegetable impressions. — Also much of an extraordinary brick red crumbling fractured unequally grained sandstone in fine sedimentary base, perhaps volcanic ashes 2716 —
A red conglomerate pebbles of crystalline rocks as big as nuts. — Such varieties were associated with the
usual white greenish, brown sandstones as above. — The lavas are of infinite different kinds, a red pseudo-brecciated crystall: rock is one. 2717 —
From the similarity in mineralogical nature of this formation to the tertiary one of Chiloe & Concepcion &c I expected to find silicified wood. On the South side of road (a), about 1/2 a mile to the East of Agua del Zorro, high up on the western slope I succeeded in meeting with this substance under very peculiar circumstances — In a low broken escarpement
(a) To describe the spot with great accuracy, proceeding eastward from the Agua del Torro, you pass on the North side of road, the remains of a Rancho. attached to some gold mines. Shortly afterwards the road passes through (
& bends at a lo gully with low but steep rocks on each hand. it here bends & the ascent is rather steeper. — A few hundred yards further on, on S. side the green sandstone & white columns are visible within stone's throw of the Road. —
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of green. compact fine grained sandstone gritstone 2698 I found 11 silicified trees & about 40 columns of crystallized carb: of lime. — The strata dipped to the WSW 20°-30°. All the shafts were inclined to the opposite point. that is ENE at about 70°. Therefore, before the upheaval the trees must have been quite upright. —
The diameter of all is nearly, the same 18 inches, being the greatest & from that to a foot. — The silicified wood. appears all Dicotylodonous 2703 ... 2708, is blackened, but very perfect, the centre of tree & concentric lines all distinct. — The longest piece which protected was 4&1/2 feet.
The cylindrical columns of snow white, coarsely crystallized carb: of lime were very conspicuous & reminded me of Lot's wife turned into a pillar of salt. There is no trace of organic structure in
lime at the calcareous spar 2709. — Yet from proximity & perfect analogy of form I feel quite certain that it fills up the place of a former tree. — There are drusy cavities lined with minute quartz crystals in the spar. 2710 — One tree was replaced by a compact hard pale brown calcareous marl 2711. one of the white columns was visible for a length of seven feet. —
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All the trees had the nearly vertical position. I saw however two short pieces as thick as a man's arm. (perhaps broken branches)
lying on the ground imbed horizontally imbedded. — It appears to me rather curious the manner in which either carb of line or silex has taken absolute possession of the different trees; the quartzose lining of the drusy cavities is to a certain extent an exception: & there are when within a few feet of each other. — Making allowance for the inclined strata, these trees are seen nearly on the same level. — They form a group: all the silicified ones, together some of the others occur within 60 yards of each other: the most remote are not perhaps more than 150 yards from each other. Some of the trees are within a yard. — If they again could possess leaves & branches they would form an elegant cluster in an open country. — The sandstone is thinly stratified, the layers being marked by slight changes in color & nature. It is evident that the accumulation has been gradual; the sandstone where in contact with the silicified shaft is marked with concentric lines, as if of the bark. 2702 —
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I saw neither branches or roots. The sandstone can be examined for about 20 ft beneath the trees. (But I omitted sufficiently to examine this important point). There was not material change in its nature. If however there formerly was a
str bed of vegetable mould, it could never be recognized, when all the carbonaceous matter had been removed or replaced by silex & carb of lime. —
In no part of the superior or inferior layers could I meet with any signs of vegetable structure.
The green sandstone is covered by the alumino ferruginous mechanico stone 2699; which passes into a fine grained purplish sandstone. 2700 — In one spot in these strata there was a most curious
variety, amygdaloid &c &c 2701 (a) —
It is possible that some of these changes may be owing to effect of metallic veins, which are here present. There is one large one 2713, near to the trees, which runs NW & SE & which has affected the neighbouring strata. Another close beneath the superincumbent lava in a brown sandstone, this is composed of an infinity of ferruginous threads, which have one general direction, but unite &
kn interlace into knots (b).
(a) In these sedimentary beds, there are veins of white carb: of lime & agate & a very few fragments of volcanic rocks. —
(b) [sketch] This will give an idea of what I mean. — The rock is blasted & altered between the threads.
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Again another in the green sandstone, running E-W which has actually been worked for gold. —
The whole sandstones may be from 400 to 500 ft thick, they repose on lavas are covered by an immense conformable bed,
from upwards of a 1000 to 2000 ft thick! (I do not say one stream, nor know that it is so) of a black compact angitic (?) lava. 2714 — To the West I could see at least 5 regular alternations, of volcanic & sedimentary deposits, each of these were several ft thick: (from these come the remarkable varieties above described) & all dipping at small angle westerly. The total thickness of matter above the sandstone with the trees must be several thousand ft; how much below I can form no estimate. —
The elevation of the place, where the trees are, must be about 7000 ft above the sea. — To account for their present position & the mass of superincumbent subaqueous deposits we must suppose, that either they actually grey, where now seen petrified, or were floated on an island & there sunk. — In the first case,
we must either it is clear the wood must have grown on elevated marine deposits or & near the edge of the ocean.
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that great subsidences
sunk lowered the land, that fresh deposits were accumulated & then raised to their present height — an amount of change certainly enormous, but yet supposed to have taken place in a highly volcanized region. —
Or the trees grew at the
base bottom of a drained lake (or inland sea) & that the mouth was closed so as to allow fresh depositions. —
Many reasons induce me entirely to reject this, it is hardly worth while to state them all.
They went If the anticlinal lines of elevation in the Uspallata range were again lowered, there would be a continuous slope of the strata to the Pampas, which certainly are marine. — the valleys plains of the Pampas blend into the valleys. — The Uspallata plain strictly resembles in every respect the marine? basins of Chili. — This latter plain also blends into the shingle terraces which ascend the valleys of main Cordilleras to a great height. — the difficulty of imagining the boundaries of any lake. before the upheaval of the Uspallata line of mountains &c &c. —
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We come then to the last alternative of a floating Isld. I
t suppose most people will think this the most probable explanation. (b) — The only difficulty. is the size its dimensions, the truely upright position & the size of the trees. — I must confess: however that I myself cannot quite banish the idea of a subsidence (a), enormous as the extent of movement required assuredly is. —
To return to the section of the Uspallata range. — Low down on the western slope I first noticed volcanic dykes: at no great distance there was a high hill, the upper part of which being unstratified perhaps has been a point of eruption. — On the whole of this side the dip has been to the West, near the margin of the Uspallata plain. not visible from Agua del Guanaco) we have
an East dip line of low hills with an East dip, a valley, some higher hills with a dip from 45° to 70° to the West, — So that there is a distinct. parallel small anticlinal line parallel to the grand range. The outer West dip forms hillocks on the plain itself. —
(a) The hypothetical case occurred to me, whether some salt deposits might not be thus explained. At the foot of the Cordilleras there are several large Salinas of pure salt in the plains to the South: if such by subsidence was covered by a stratum of sand, & these continued to accumulate, would not the pressure, percolation & perhaps subterranean heat convert the crystallized salt into rock salt? — The form of these would be plano-convex, but the curvature of the bottom in one of these large shallow salinas would scarcely ever be perceptible in a mass of strata. —
(b) De la Beche translation of Annales des Mines p. 208 Brongniart states that vertical trees are very common. — It is Mr Charpentiers suppositions of floating islands & vertical submersed trees. —1
Here In same work M. Brochant has good description of gypseous formations of Alps. — it would be worth comparing.2
1 De la Beche 1824, pp. 208ff is an article by Alexandre Brongniart 'On fossil vegetables traversing the beds of the coal measures'. The observations of Charpentier are noted on pp. 215-216.
2 Brochant De Villiers article, 'Observations on the formations of Ancient Gypsum occurring in the Alps, particularly on those considered as primitive; preceded by new facts relative to the transition rocks of that chain', is in the same volume, pp. 62ff
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The road having nearly crossed the mountains, bends towards the Estancia to the South. The valleys from the plain of Uspallata enter this range with a broard level surface, in the same manner as the Pampas do. — In the mouth of the valley, by which the road descends, there is great confusion in the stratification owing to some oblique lines of elevation.
One runs about NE & SW, appears owing to grand dyke or rather chain of small porphyry hills. —
(Porphyry with numerous cryst of feldspar & quartz in a sparing basis) 2719 — which have burst up through the strata. — To the South, much of the strata were vertical & all highly inclined. —
The landscape was most extraordinary. I have already mentioned the various colors of the sedimentary deposits & lavas, to these were added hills of lilac purple & red porphyries. — Still traveling South, on the margin of the plain, there is a band of hills of a sonorous brittle glossy coarse grained clay slate. It is laminated 2720 & the dip nearly vertical to the East. — The clay slate is much altered, & is in parts crystalline & feldspathic, like (2723); contain
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veins of quartz: crossing inwards a N & S ridge the slate became green & purple & was succeeded by a band of soft purplish clay-stone porphyry 2721: this was associated, without any order with large veins & angular patches of a white earthy feldspathic stone 2722. The junction of the two is nearly straight, but I did not see point of contact. —
A little further to the South & to the East of the houses of Uspallata in the valley of Canota we meet similar rocks, exposed in a very extraordinary manner. V. ground plan — The valley is broard & flat, in the middle there is a group of several little hills of porphyries, of several distinct kinds; dark-colored, lilac, yellow & white, of which specimens may be seen 2724 2725;
These have burst up (a) through the slate or through the fragmentary mass of angular pieces of slate, cemented together: some of the paps are thus capped & cased. — I saw one mass of the porphyry (2724 & 25) reposing at ∠70° against one of these fragmentary masses. it sent a small vein into it. & its whole side was marked with the angular points of slate, which could either be extracted or would break off. — I
(a) Some of the porphyries have a broken or brecciated recemented structure
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can not imagine a prettier proof of its formerly fluid nature & its position showed its injection.
Till I saw this I suspected the fragmentary mass had been collected subsequently to the emission of the porphyry. It is hence
certain probable that this has been a point of erruption, through a surface strewed with fragments or composed of the solid parent rock. — To the North we have the altered slate & to the east West small hillocks of the same, all with a N & S cleavage. — To the South there is a large triangular mountain of slate, on through which one or two points of porphyry protrude.
The slate is much altered & is a greenish or grey feldspathic rock 2723: the strata of greywacke can still be distinguished; there are veins of quartz: the cleavage is much twisted, yet the general direction is NE & SW instead of N & S, this is probably owing to this being a point of upheaval. To the NE there is a grand escarpement (painted like a geological section) composed of lavas & intermediate sediments, which dip from 50°-60° to th NE.
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(N) (E) (S) (W)
Valley of Uspallata (much broarder)
Road to Hornillos
SW Line of porphy dyke NE
Line of section
Highly inclined stratification
Pass of Cañota
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To the South there is a corresponding escarpment which dips to the SSW at 70°. Hence the two lines converge; the cause of this appears to be owing to the triangular forms of the elevated & altered slates. In the enclosed space there are some porphyry paps, which belong to the North & South band of emptive points.
I will now describe some of the lower part of the NE Escarpement, the upper strata I did not examine. — On the slate itself on a fragmentary mass, as before composed of slate, there is (1st) a mass of hard red breccia, containing much slate; (2d) a pale purple porphyry (3d) dark blueish stone with acicular cyst of glassy feldspar & olivine? structure sphaerical. 2726 — (4th) a great mass of purple porphyry containing those abrupt curious masses of white Feldspathic rock, precisely like the eruptive porphyry (2721: 22) (5th) thin strata of greenish white friable aluminous sedimentary substance 2727 containing extraneous particles: an oblique dyke crosses there. — (6th) grand quantity of cream-colored calcstone porphyry. imperfectly columnar, divided obscurely by three
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parallel lines of partition (7th) another grand mass of lilac & white porphyries. — I believe some of these different
P strata are separated by seams of sedimentary deposits. — The porphyries exactly resemble those of the injected hillocks, but are unquestionably lava beds. — The slate & greywacke is the same as in the Eastern slope of the range, perhaps, a part of its altered nature, as well as upheaval may be owing to some granite point beneath.
The lavas here are different; but variety seems their character. The predominant proportion of crystalline rock to sediments, shows that the source of the former is not far distant: from the close resemblance in mineralogical nature of the lavas & injected hillocks I believe some of these to be bases of volcanoes. — It must however be remembered, that, but a short distance to the North there are porphyries, posterior to all the strata, & it will afterwards be suggested, that the main source of the Uspallata lavas is in the East slope of main. Cordilleras. —
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The fact of the fragmentary mass either covered by lavas or burst through by porphyries induces me to
believe think take the opinion that the slate existed as a formation previous to all the above strata. — The whole of this neighbourhead is metalliferous; in the triangular slate mountain there is a curious one of silver 2928.
The famous silver mines of Uspallata are said to be situated in clay slate: they are to the South of the section but north of the Canota pass. — The one is argentiferous lead. 2729 — There is also gold ore like that of Hornillos. 2730 — It appears to me probable that all these metallic veins pass through & are connected in their origin, with inferior granite.
The chain of Uspallata is separated from the main Cordilleras, by a narrow level plain, it is said to extend at least 180 miles to the northward —
it the chain is in many places highly metalliferous. San Juan. Fanmatira. Lipez &c (Miers Chili). From the same authority, there are mines of sulphur & alum slate; probably there is an ancient volcanic forces, as in the East slope of the Portillo. — I also heard of silicified wood, near S. Guasco
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Mr Miers was also informed , that coal has been found in these mountains & it appears certain that a little way South of Mendoza there is a spring of mineral pitch. —
With respect to the plain of Uspallata its description & discussion concerning its marine or lacustrine origin, will hereafter be given. — I have now only to mention that in parts, layers of white & bright red sands, slightly agglutinated & containing lines or beds of pebbles dip at angle 45° or more to the West. — There are some irregularities & trifling faults. — The pebbles from size of nut to large apple are altered slates, porphyries, & much granite & protogine porphyry. — They may have proceeded from either side of plain, from the two last kinds of pebbles I suspect from the main Cordilleras
The inclined sands is in places covered by thick mass of shingle, horizontally stratified & evidently on aqueous regular deposit. — The westerly dip must form part of that extreme line of hills, described in the section. — V [Journal] [illeg] of tilted gravel in Scotland — This is the first time I have ever seen tilted alluvium!! its presence is perhaps owing to the protection of the horizontal & subsequent mass. — These
[illeg] facts also, proves render it probable, that in the Uspallata mountains. received a large
Edinbur. Phil J. 1838, p. 384 in that Paper woodcut
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the strata received nearly all their inclination when beneath the sea. — We know from the horizontal shingle (which
with is marine) that the last 6000 ft of their elevation has added without great distribution. (height of plain)
The occurrence of the inclined alluvium also points out, that some change in the surrounding circumstances, must have happened previous to the upheaval, so as to account for such depositions in place of the regular fine strata. — Was this the upheaval of the central & highest anticlinal band of the Uspallata mountains? —
We now begin on the description of the main Cordilleras: the grand valley of the Rio Mendoza first enters, in a Southerly, then SSW, SW elevation to the junction of the river of Tupungata, where the line is nearly E & W. —
At the mouth of this valley, the mountains present a grand & flat topped mass, cut into square blocks by the different ravines. — They consist of an extraordinary mass of differently colored porphyries: the greater part appears injected
amongst one mass into another. — thus some however is obscurely stratified the lines dipping at a small angle to the East.
The mountains are traversed by vast numbers of great dykes, which assume every form
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intersect, each other. the greater number are red. —
On a nearly precipitous side of mountain, the higher part behind was white, & the lower in front of blackish porphyry.
two immense dykes were injected downwards at ∠45° & which divided & thinned out. — This I suspect is a rare case. — Further to the West a ball of white porphyry
to is incased in some strata. within this there is a pap of broken & recemented dark purple porphyry & the two are traversed by grand greenstone dyke. — (The whole has been elevated by granite)
We see then three distinct injections on the same exact point. This is important as illustrating successive elevations of strata. — How are we understand the variety of melted matter beneath the same part of surface?. — Some way within the valley there is a magnificent group of lofty peaked hills of a peculiar porphyry. This has a red basis porphyritic with large crystals of quartz 2731, red feldspar & bits of green matter; it exactly the protogine of the Portillo, the whole substance not being separated into distinct crystals: I shall call it Protogine Porph: It is almost universally the most inferior of the porphyries, having broken through all the superior & older ones. — Where in
(a) From extended observation I must reurge this fact. — I may point out the analogy of a volcano which
puts pours out from the same source at different periods very different lavas. I feel certain that frequently when any mass of strata are much traversed by dykes, they will be found by intersection to be of different ages & from the great difference of their mineralogical nature perhaps of widely separated intervals. — Even when they do not intersect each other, but are close, I cannot believe two dykes entirely distinct in their constitution, can have proceeded at one time from one source. —
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one place in contact with a white kind; the latter was broken into a mass of fragments, recemented clearly by the action of heat. In
some places a few cases however the porphyry was traversed by white dykes, (which very likely proceed from the doom if inferior granite): — This These porphyries are of course generally without stratification, yet on the South side the upper parts seem to blend with a stratified mass dipping East. —
I cannot doubt but what these grand masses of different porphyries are the bases of numerous submarine volcanos, from which the stupendous streams of the Uspallata lavas have proceeded.
Beyond the pass of the Jaula I first recognized the porphyritic breccia; much of it was so much altered, that it could not be distinguished from the injected porphyry & that which has flowed.
It may sound strange, but I feel sure, that in the Cordilleras specimens of porphyries, produced
by in these three distinct methods manners frequently could not be told apart. — The injected porphyry & lavas have both sometimes a brecciated structure; but it is quite different from that of the porph. breccia; the stratified structure, the more or less rounded
[sketch] [test] this
The tilted alluvium in Uspallata plain proof that entire chain elevated before flank one. — ∴ also Uspallata beds posterior to the elevation of first range containing great gypseous division. —
Mr Bynoe on currents in breaks. RN p 1411
Lava in Cordilleras & on Eastern plains by Antuco
Athenaeum April 1836, p. 3022
Excellent view of mica slate on Portillo range
R N p 150 & 151 (on valley ↔ & 155
Except from trees not trace of subsidence in Uspallata
Arrowsmith account of Himalaya penetrated by rivers3
See Molina account of deluge for earthquake4
Greenough on unequality in mineralogical nature of formations, on opposite side of mountains, p. 545
1 Red notebook, p. 68.
2 Anon 1837. Darwin misdated the reference as 1836.
3 The same reference to John Arrowsmith occurs in Notebook A, p. 2e.
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form of the
pebbles fragments, their different varieties, the evidence of sedimentary deposition, in the linear arrangement of variously sized particles; the association with claystone & jaspery rocks not crystalline. — are all proofs of this. — The strata of porph. breccia are here traversed by porphyritic greenstone dykes, which class of dykes appears common to this formation, perhaps owing to fusion of the inferior slates. — The strata here also alternate with some white beds. —
The strata appear to dip
in from each side of a SW & NE line of the valley. — The escarpements are so broken by dykes & masses of porphyries, that only remnants exist. — At the pass. (where mules are generally unloaded) there is a grand hill of the porph. protogine, which has burst through this formation. — From this point to the R. de las Vacas we have a clay slate formation at the bottom of the valley capped by the porph: breccia.
At first it resembles that of Uspallata, has vertical contorted cleavage & quartz veins, it soon becomes more crystalline & blacker 2733 & passes into true greenstone with some grains of quartz: the upper parts are composed of one of those greywacke-like conglomerate
(a) Even when in the strata, in some spots the cleavage can be traced, running about NW & SE.
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where the cement is of the same nature as the fragments 2732 — in a hand specimen it would be supposed a concretionary semi-cryst. feldspathic stone. —
The slate is traversed by some dyke & masses of a soft decomposing white rock. — after passing for 2 or 3 miles
through along this formation (the line of road - strike nearly parallel) we come to an axis of true white granite. — This underlies & protrudes itself amongst the feldspathic slate. I examined part of the junction, a mass of fragments of crystallized greenstone, were united by a mesh-work of granite veins. It is precisely the same as the porphyry & granite in the Valle del Yeso in the Portillo. —
I believe the salte is the same as that of Uspallata; in position it
f exactly corresponds to the greenish irregular mass of hills, described in the West mouth of Valley of Maypo, as separating the granite & porph. breccia. — Indeed I may generally observe that the lowest beds at the fort of West slope are green porphyries & feldspathic rocks, which might have proceeded from the more complete fusion of this substance. —
the granite itself (a) is traversed by a green dyke, as far as I remember any dyke in the granite is a rare occurrence. — The strata dip from each side of the
(a) near junction I picked up the mineral (2734)
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mass of granite; those having the SE dip have been described.
on the opposite side similar ones irregularly dip to the NW. — (I must shall afterwards explain, that this appears rather a basis of disturbance than regular line). —
From the Rio de las Vacas to the union of the rivers
these sl the altered slates of porph. breccia are traversed by white porphyry dykes irregularly stratified; near the Casucha of Pujios granite is again seen & here we have a alternative dip to the SW. — the direction of the dip gradually changes till at Puente del Inca it is W by S & the angle about 30°. — Here we have a grand line of elevation which runs through the country in a N by W & S by E line; some of its peaks nearly rival in height the main ridge. —
With the exception of the basin with granitic centre, above described the stratification to the East of this line can be seen both to the North & South horizontal. These
lines strata evidently goes on are continuous to the [west] the flat topped porphyry formation on the margin of the Uspallata plain. It is very rare to see such lofty mountains in the Cordilleras with horizontal strata. — At the Puente
The Within the very line of this principal chain of hills near the Inca's bridge, there are several very copious saline hot springs, from which large volumes of gas are always escaping. Lieut. Brand1 in his passage of the Andes I 240 gives their temperature as true being 91°. — one 83 & the coldest 66°. — From Miers table the elevation must be nearly 9000 ft —
Mr Brand took some water which was enclosed by Mr Brand (the chemist) 10 cubical inches contained 45 gr of matter. Contains greatest proportion of salt, gypsum, carb of lime, iron & sulphuretted hydrogen & carbonic acid in excess. Its position with respect to stratification calls to mind the similar fact at Cauquenes: also at V. Vicenca, an anticlinal line probably exists. I may remark in each three cases that these are springs of different temperature.1
NB. The springs deposit much tufa: where the water drops from height, heaps of concretions are formed from the size of small shot to a small apple. (2759) 2759 These have curiously been mistaken for Bezoar stones from the Guanaco (V Miers). They are found in a place where guanacos could hardly descend, whilst in the upper springs, which are easily accessible, there are none. — They have been collected as a superstitious remedy as bezoar stones. —
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I examined a section of the strata I should suppose nearly
6000 5000 ft thick. (1) — The lowest bed which I actually examined. is reddish purple base porph. 2747 with cryst of feldspar, it is very obscurely brecciated (a): in its upper parts contains a few small pebbles: the bed is thick, is divided by some more strata of the same coloured rock, from the greenish feldspathic slate, which it covers conformably. & is I believe the base of all the formation in the Andes. —
2d On this porphyritic breccia, we have 80 ft of a dull whitish very compact, hard heavy aluminous-calcareous rock, in the lower parts it is a recemented breccia, & bands of a similar substance are found in all parts. — The rock is much fractured, all the weathered surfaces are bright red, especially low down nearer to the porph. breccia. — There are obscure marks of shells. —
3d A red, quartzose fine grained conglomerate, with white patches 2748, appear owing to a sort of crystalline process one spot is traversed by
white hairs from red basis; This rock will be found much more compact & altered than any of the superior conglomerates.
4th A whitish charty limestone as before, with nodules of bluish compact, irregular fracture aluminous limestone 2753
5th white conglomerate, many particles of quartz, almost blended together, but basis not at all porphyritic or crystalline. —
(a) There are some hair-like veins of cryst. carb of lime. —
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6th white fine sandstone, so very quartzose & compact as almost to deserve name of quartz.
7th & 8th A red & white bed not visited from want of time
9th Yellow, fine grained thinly stratified limestone, effervesces slowly: does it contain magnesia? (2735) drusy small cavities, lined with cryst of carb. of lime; some of these have figures of shells. — On the plain, at the foot there were numerous fragments of a white cryst limestone, with many fragments of shells (2750 51: 52) amongst these gryphites could be distinguished & one perfect oyster. Also a few lines of small white & quartz pebbles
10th Pale lilac feldspathic base with numerous cryst. of white feldspar & black mica. 2736 — no signs of violence, conformable 20-30 ft thick — a lava
11th yellow limestone, as in (9th), part stained purple, compact.
12th Greyish purple stone, composed of little specks of white carb of lime 2737, an earthy mineral, red crystals & amygdaloid with carb of lime. (a) — I have seen similar rocks in North of Chili. — do not know origin. —
13th Red sandstone covered by grand bed 300 ft thick of a coarse hard red conglomerate, cement calcareous 2738, pebbles all red porphyries, excepting a few quartz from size of nut to mans head. — This is the coarsest rock I have seen in this part of Chili. — In its middle there is a thin stratum of some white rock.
(a) Is the effect of subsequent heat, I believe all these strata have suffered from this cause: heat acting on more susceptible material or a lava!?
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14th grand bed of feldspathic lava 2739, which forms from its hardness a projecting wall; upper parts slightly cellular containing little patches of ferruginous matter. (a) — Black mica, white cryst of feldspar. — This perhaps is about 1/2 way up mountain about 3000 ft high
15th Dull purplish fine grained calcareous cryst aspect sandstone 2740
16th This passes by a coarser red sandstone into a very coarse white conglomerate, base chiefly particles of quartz
17th Alternations of red conglomerate, (sandstone as 2740) & much of that curious rock (2737)
18th A greenish stone, somewhat similar to (2737) 2741
19th Many thin strata of compact or more compact pale purple, fine grained slightly calcareous sandstone 2742
20th Pure gypsum 2744 45: 46: Bed about 300 ft thick: resembles in every respect that described at the Portillo: there are the same hard & large nodules, it
alternate contains in layers the black crystals, & calcareous matter
21st Red sandstone (about same thickness) same as (2742)
22d Layers of pure gypsum, with other purplish impure & aluminous & rather hard. 2743 — Layers thin, even as thin as paper, some curiously convoluted as in specimen. —
24th The red sandstone all very thick strata
25th & 26 at a great height, but did not reach them same again gypsum, covered, by the alternating layers of pure & impure gypsum
(a) The thickness varied a little different parts.
1835 Uspallata Pass 75
All the neighbouring & most lofty peaks are composed of these gypseous strata, the total thickness of the alternations cannot be less than 2000 ft. — I distinctly noticed that all the sandstone, conglomerate & gypsum beds above the main lava are subject to infinite changes in number order & thickness. A thick stratum of
gypsum sandstone between two of gypsum would in a short distance thin out to nothing & be replaced by gypsum. — The reverse also in places happened; So that one end of mass of strata might be all sandstone & the other all gypsum; the junction being by a mortice dovetail work of acute wedges. — I believe a bed 300 ft thick would thin out in a mile. —
A little way to the West, part of the gypsum was changed into brown, ferruginous hard calcareous sandstone 2754, & contained some little quite black, with ferruginous veins, something like that of the
Portillo Puquenas ridge. — In the whole section including the lower rocks, there would appear to be but little continuity in the individual strata. — yet from this a good general idea may be imagined of the formations. I feel little doubt that the porph. breccia, near the granitic centre
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are parts of the lower conglomerates, in the section, only more altered by the nearness of granite. The general appearance of the included pebbles was the same; but the basis far more porphyritic. This is
a not the only instance, where a difference in the porph: breccia, could be clearly accounted for. — The gryphites in the limestone are the same as in the Portillo: here they are immediately below it: whilst these actually within that formation. — The valley here runs nearly due West, as the strata dip to that point, there is an excellent section (not [illeg]). The general inclination is about 30°, in some places even 45°. They are traversed by a number of parallel ┴ [it] which reduce the aggregate dip <excised section>
& sometimes but rarely destroys it. very <excised section>
From the Puente del Inca to the Cumbre or pass of main ridge the formations are similar, but stratification rather complicated. V Section ┴ I have drawn the sections on both & North side of the valley, which are rather different. —
In both (B) is the mountain which has been so minutely described. (a) is the lofty table mountains seen in the distance (ax) is the basin
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in the valley, which the pap of granite has disturbed. — Travelling West, first on the North side, the gently inclined strata of (B) are abruptly covered by a fault where the inclination is 70°. — This forms one side of the Valley of R. Horcones; on the West side of which, we have the same
fo gypseous formation still inclined to West but at angle of 45. — This is partly explained by the forms in the mountain tops to the North, we there see a synclinal basin & a lofty mountain through the middle of which there is an anticlinal line; it would appear as if all the synclinal strata on the West side of axis c had been removed by the excavation of the broard Valley of Horcones: the two D° correspond. — From the fault in (B) the synclinal line was here probably more abrupt & deep. —
We then come to a hill (E) with the strata dipping from 60 to 80° to the SW. it is an oblique line & is owing to grand. mass of jagged lofty (I believe injected) dark colored porphyries. (a) To the westward they are connected with clearly stratified porph. breccia strata curved. dip high angle to the West
(a) This is near Las Cuevas, a grand mass of alluvium, piled up in singular forms composed of large blocks of the dark crystalline porphyry, has by some travellers been rather laughably mistaken for a crater. —
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by some are traversed by many very grand dykes of white feldspathic porphyry, which very probably has come from the inferior syenitic greenstone. —
In the section South of valley, the strata of (B) are prolonged by the means of faults as described: the western end (near E) is entirely gypseous. — At the point (E) there is a small curvature in the lines, this is owing to the prolongation of the oblique line of violence of E on the North side. — At (G) there is porph. breccia & limestone dipping 45° to the West: then a line of great movement at (z). In the distance a hillock is seen with vertical stratification. — The stone of (G) is not nearly so porphyritic as in the North (G) to which it corresponds; owing to its being more remote from the igneous action & point of violence. —
These (a) are succeeded by a mass of nearly vertical (G), & most singularly contorted strata, composed of sandstones & conglomerates 2755: much of the former calcareous & of a red color. — They are traversed by a vast number of vertical & convoluted dykes
(a) This mass of sandstone lying between the porph. breccia, appears like those immediately beneath the gypsum: it is certain that some great & unintelligible dislocation has crossed the mountain
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of green, yellow & red colors; the prevalent dip is westerly, but I do not believe they lie in their proper above (G). There has been some great violence, & which to the North has determined the excavation of the extensive valley of R. de las Cuevas; the road passes over the Cumbre in the upper margin of these strata. — The peaks (J) of this central range, are composed of dark purple & whitish porphyries, they show obscure westerly dip (b), & appear to overlie the strata (H). —
We now begin to descend (a): From the Cumbra of the Casucha of Juncanillo, the jagged peaks are all dark green & reddish porphyries: where it is difficult to determine how much has been injected, flowed or metamorphosed. — At Juncanillo, from the depth of ravine, some syenitic greenstone is brought into view. — On the tops of the mountain, perhaps there are some remnants of the gypsum formation dipping West. — From this point to the Guardia Vieja; the lower parts are porphyries
su without stratification, supporting strata of various whitish, purple red, harder, softer, more or less coarse &
(a) For these few days I was not very well & could not ascend the mountains to examine closely their composition. —
NB. I have heard of anthracite having been found in the porph. breccia in valley of the Maypo. —
(b) All the central ridges, (with the exception of group of porphyry hills mentioned before) run nearly N & S or N by W & S by E.
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porphyritic breccias. — Much of the porphyries, from analogy I believe have flowed as lavas: all such as specimen, which is abundant. 2757 —
The strata all dip about 25° to W. in parts however rather to the NW. & which has been instrumental in forming the valley. —
A little below the Guardia there is a mountain without stratification; then the porph: breccia as before, traversed by many dykes, some of which are white. (a) — Universal dip without any exception on this side of the Cumbra is Westerly & not higher. — We then come dark rocks unstratified based on extensive mass of a white feldspathic porphyry, which passes into & is closely connected with syenite greenstone, containing a very little quartz.
This porphyry 2756, has a granitic aspect, cleavage contains black angular fragments &c &c, it bears to the syenitic greenstone, precisely the same mineralogical relation, which the protogine porphyry does to porphyry. —
In the mouth of the valley, the rocks are chiefly very porphyritic breccia, frequently of a green color & porphyritic greenstones: amongst these I saw one small granitic pap. —
NB. The section is supposed to end, & a little way above the Villa Nueva de St Rosa. —
(a) M. Gay in the Cordilleras of Cauquenes describes large white veins & supposes them quartz. — From the number of boulders of granite & syenitic greenstone I feel such a formation exists somewhere & it appears to me probable that these white veins
of are feldspathic porphyry. —
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The section here represented, is I believe about 70
[illeg] nautical miles: is drawn as 40 inches. (∴ 1 inch mile = .57 inch). — The highest peak is central ridge is assumed as 13500 ft above the sea: it is drawn as one inch = 3000 ft; this is about three & a half time too great in vertical proportion: The real scale is shown. — Edge of Pampas assumed 4000 ft. — Mouth of valley in Chili 3000 ft.
The accompanying chart has been copied for the proportion of distances & Miers for the heights. —
<bottom of page excised, now in 546A below>
(a) N. B. The gypseous formation is drawn too short (in horizontal extension 1 by 2/3 of inch: The porphyry to the West ought to be contracted. —
NB. Both this & the Portillo section; should be more strictly denominated a view of each side of the valley or imaginary, where the valley does not run straight. — And it must be born in mind; that it is impossible that the section should literally be <section excised>
<section excised> crossed the <section excised>
<upper half of page excised>
— Reflecting over the whole account, it seems almost certain, that the Uspallata range has undergone a great angular elevation subsequently to the main range. — In this respect & in geographical position it agrees with the Portillo line. — It will be seen there is a great & general difference in the formations which cover the altered slate in the Uspallata & main range. — From the analogy of all the tertiary formations in S.America, the Uspallata, must certainly be Posterior. — This will appear more probable.
<upper half of page excised>
a straight E & W line.— We [crossed the] <section excised> Uspallata range considerably to the north of the main range. — here it is made up of short parallel pieces viz. (W [sketch] E)
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when it is known that in the North of Chili a great mass of strata containing much shingle & silicified wood overlies conformably the grand gypseous formation. This latter, in this & the Portillo section forms the highest beds; hence we can easily believe such being elevated, the superior ones would be accumulated on the flanks of the chain: & this (conglomerate) we know to the case in the Portillo, nor have I any doubts respecting the Uspallata range. — The only fact which appears anomalous, is the absence of the older strata in the clay slate of Uspallata. — May we conjecture, that it had a previous tilt?, does it not correspond to the mica slate & clay slate of the Portillo range? —
Comparing these two sections; there is a general resemblance both in nature & superposition of the formation. — In the granite.
porphyry (partially in the slate) porphyry. porph. breccia: & gypseous formations. — In the last but one impure limestone & quartzose sandstones are
(a) We here see distinctly that the gypseous formation is conformable to the porph. breccia, a fact which I had before only presumptive [illeg] of.
(B) If the fact of the vertical trees is explained by a subsidence of the land — the absence of the intermediate strata is more easily understood for they might have been elevated & so have suffered degradation. —
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interpolated: Not that I am at all certain they might not be found in the Portillo. —
We have seen in what an extraordinary manner (a) the gypsum, conglomerate & sandstone replace each other. it is therefore the more probable that the mass of Black limestone of the Puquenas (overlying the grey sort with gryphites as here) replaces the red sandstones, great as the change may be. — We have already pointed out the analogy between the Uspallata & Portillo line: in the latter the formations are coarser (mem: the coloured band of strata above the laminated sandstone) & do not alternate with subaqueous lavas.
The conjecture which I made that a solid doom of granite lies beneath a broken crest of strata in the Portillo, is here still more probable, from the greater number of points where it appears & from the very close mineralogical similarity
of in its nature in all places. — The stratification
(a) I may observe, that to produce extensive deposits of gypsum, sandstone & conglomerate (of crystalline, chiefly red porphyritic rocks) there must have been some neighbouring coast. — Probably small volcanic islands;
or even suba. — The exact position of which of course it is now impossible even to guess. —
1835 Uspallata Pass 84
in both cases obeys the same general laws. — I think about 10 principal (excepting small faults) lines of dislocation occur. — In parts for instance West slope from the Cumbre, amongst the porphyries it is nearly guess work. — In the Portillo about the same number are conspicuous; they are placed however in different relative positions. —
I cannot help admiring the beautiful symmetry of the Uspallata range: the granitic peaks may be considered rather as accidents, which serve however to point out the cause in an arch of the same substance at a greater depth. —
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Citation: John van Wyhe, editor. 2002-. The Complete Work of Charles Darwin Online. (http://darwin-online.org.uk/)
File last updated 2 July, 2012