466

1835 March 18^th to April 10^th. St. Jago to Mendoza (1)

In my passage of the Andes by the Portillo pass & return by that of Uspallata, I will describe the geology in the order (for the want of a better) in which it occurred. —

I must first remark, that in my paper on Chili (p 14 & 15 &c) I have described a great mass of strata under the name of porphyritic breccia. These rocks are of a red or purple color, stratified, contain certain marks of a mechanical origin, yet are more or less porphyritic in their structure: (The included pebbles or fragments are almost invariably porphyries.) —

[in margin] Should basal porphyries be calledPlutonicor Volcanic

The whole rock often passes into a true porphyry. This description is the explanation from inferior heat I believe to be quite correct. But from what I have subsequently seen in the North of Chili, where the metamorphic action has not been so ac strong, I think it highly probable that some of the true porphyries are subaqueous lavas, & that the pebbles come from the action of the sea, on the higher parts of these streams.

I am sure such is the case in the North part of Chili (a) but feel little sanguine at not having discovered this fact in the lower parts of the formations near St Jago in central Chili. —

This remark must be borne is mind in reading the account of the geology of Chili: for instance, the lower great mass of perfect porphyry in the Bell mountain of Quillota, very probably has been a lava stream. —

In a similar manner much of the porphyry in the Andes, which in my note book I have designed "entirely metamorphic conglomerate"¹ i.e. true porphyry, may most probably be attributed to this origin. of course hills of injected porphyry, without stratification are generally very distinct from (but not in mineralogical nature) the porphyries conformable to sedimentary strata.

[in margin] V. Back of page 68

Urge clearness of atmosphere no vegetation steepness I learnt more than in as many months on the [pampas]

Should basal Porphyries be called Plutonic or Volcanic

¹ A reference to the St. Fe notebook although this wording is not found there.

466 verso

(a) I recollect distinctly when examining the porph. breccia putting the very hypothetical case of a subaqueous lava, & coming to the conclusion that from the extreme metamorphic action it would be scarcely possible to distinguish the different kinds of origin. —

N.B. Where formerly in Chili paper I have not distinctly traced the connection of a perfect porphyry: with a mechanical rock it will be rash to suppose the former owes its origin to the later metamorphosed. —

Greenough p. 54¹ on difference of mineralogical nature of opposite sides of mountain explained by the Portillo section

Geological affinities of gypsum p. 390
Daubuisson vol. II²

Metamorphic action crystals produce. A. p. 15

[sketch across hinge of sheet] Plains of St Jago Granite (E)

In discussing cutting through of higher range by streams from lower country. — mention Copiapo withTiticaca. — if elevation too quick for erosion: then lake formed

Von Buch mentions cavernosity in gypsum.³ Vol. VI p. 631 Humboldt

Supercretaceous conglom. upheaved from lime of Righi (as ∴ two elevations in one line. De la Beche. Manual p. 199 — & 207⁴

NB. Look at first page of recapitulation

RN p 63 about terraces in valleys⁵

¹ Greenough 1819.

² Aubuisson de Voisins 1819.

³ See Humboldt 1819-1829, vol. 6, II, p. 631.

⁴ De la Beche 1831.

⁵ Red notebook, p. 47.

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1835 St Jago — Mendoza (2)

Beginning (a) then by the Portillo pass; in the very mouth of the valley of the R. Maypo, the first rock I examined was in a little hillock composed of syenite. This was succeeded by an irregular band of low hills, running about N & S, composed of a white granite with black mica.

In the neighbouring mountains, a great mass of stratified red rocks belonging to the porph. breccia group could be seen superimposed on the granite hills & dipping from them at a small angle to the East. Now Formerly near St Jago, & now a little to the South, of it, at the Almeira I noticed & was surprised by the external face of the main range presenting the edges of strata dipping internally.

[sketch] Plain of St Jago (E)

We may well understand this by imagining that beneath the plain at the foot, there is a range of granite hills, which in the mouth of the valley of Maypo has been exposed. — My surprise was owing to the expectation that the exterior face of the grand chain, would show an anticlinal slope from some central line of disturbance. — In following up the valley the road of course crosses the different formations, I found interposed between the granite & porph. breccia, a rough tract of semi crystalline greenish feldspathic rock, which resembles the "altered slate" of the Bell mountains; parts however were more crystalline & darker colored & contained delicate crystals of feldspar: there was also a mass of highly cryst.- porph. greenstone, the origin of which is probably distinct. — Where we entered the porph breccia a good deal of it was tinged of a greenish color, & very perfectly porphyritic, the traces of the mechanical structure

467 verso

(a) I should have begun by stating that "The Andes in the pass of the Portillo, consists of a double range; the first or Western is perhaps nearly in not far from the centre, is called Puquenes, which & divides the waters. — The other ridge the Portillo is perhaps of equal or greater height, it is separated from the Puquenes by an undulating country hilly district which gently slopes to the East; the water of this intermediate country, find a passage, some distance to the South of the road, through the Eastern chain & reach the Pampas. —

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1835 St. Jago. Mendoza (3)

were frequently very obscure; yet amongst such varieties I saw a conformable band of coarse breccia & conglomerate = about a foot wide, the edges of the fragments almost blending together. This is one of the cases, where perhaps a layer of conglomerate has separated two streams of Lava, or it may, as I formerly imagined be simply owing to a greater mechanical resistance, from size of fragments to complete union & compact crystalline structure. — This formation extends from this first point to a little below the junction of the rivers del Valle del Yeso & Volcano, a days journey; as however the valley runs for the greater part of the distance very southerly & at last only SE. this must by no means be supposed its thickness in the transverse section of the Cordilleras; for the same reason it is nearly impossible to understand the stratification; the inclination generally is not great. I saw only in one spot a dip of 50°. the direction of the lines is perhaps more generally N. & S. but there are many irregularities, & twice I observed well developed mantle-shaped strata. There were some few dykes. — # A little below the junction of the R. Colorado, there is a remarkable hill, of a white earthy decomposed feldspathic stone 2598, contains bits of chlorite & has obscure tendency to crystall structure in parts; — further up there would appear to be a transition from the above through some pale green porphyries, 2599 ... 601 into a true compact green porphyry: as far as I could see these rocks overlie conformably the lower porph. breccia. stone as With this exception, all the rocks, as seen in the bare & steep sides of the surrounding mountains, were reddish, brown & purplish porphyries, associated with

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1835 St Jago — Mendoza (4)

the crystalline metamorphized mechanical rocks. The whole mass was almost invariably thinly stratified. more compact porphyries (2602) 2602 with obscure breccia structure being separated by thinner layers of less crystalline crumbling varieties. Some of the breccia conglom was very coarse the partially rounded fragments of porphyries being from 6 inches even to 12 long. — in parts these were so distinct. that they could be separated, without breakage from the matrix. — This latter chiefly occurred in the higher beds. — In one spot only I saw porphyry with a rough columnar structure. — The thickness of this formation must be very great, as far as I could judge from appearance it reaches in this neighbourhead to the highest summits.

I believe (a) the mines of San Pedro de Nolasko are situated in the upper strata: as the inclination is not great I believe the strata must be from 6 to 7000 ft thick, the same conclusion, as I formerly drew at Japuel near Aconcagua. — Near to the forementioned junction of the rivers Yeso & Volcan: we are in the midst of a range of white conical hills of granitic rocks.

The most abundant variety 2603 is a mixture of white feldspar in foliated crystals & green hornblende, with some chlorite in a not neither in a well crystallized form. — As this rock is the extremely common in whole line of Cordilleras from S. Fernando to Copiapò (& in a cabinet at Lima) I will particularly describe it. Being composed as above 2603 it rarely assumes a few grains of quartz & sometimes black mica, in these latter cases it becomes a true syenite & frequently passes into a well developed granite.

469 verso

(a) The ascent to these silver mines is described by Capt. Head:¹ they are situated nearly at the limit of perpetual snow. the one occurs apparently in a kind of granite base specimen (2758) is very poor. — The mines are notorious from the large sums lost in working them. —

¹ Head 1826.

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1835 St. Jago — Mendoza (5)

although far more abundant it is generally associated with some granite & has the same geological position.

In the first aspect of the stone, in the form of the hills, in the kind of cleavage, in its minerals (chiefly epidote & the black stellated one of Yaquil), it precisely resembles granite.

Again there is another similarity in the heart of the stone, small angular patches of a blacker variety are contained, which, whatever their origin may be, appear as entangled pieces of some foreign rock. —

Whatever a mineralogist would call this substance rock stone, it is in fact a syenite but as it does but seldom contain quartz, I shall always use the name of syenite greenstone.

In this individual case this rock, contained some little quartz & black mica, & was associated with some true granite, the latter, in places passing into a fine grained kind with groups of cryst: of (hornblende?) 2606

The formation appeared to run about SE & NW & was surrounded on all hands by lofty hills of the purplish stratified rocks, which dipped from it at a small angle. — The granitic rocks, one one side are immediately capped by a heavy, blackish green hornblendic rock 2604 2605 & by another compact purplish finely crystalline one.

I do not know the exact connection of these with the granite. — At a point round which the valley doubles a little way beyond the granite, we again have the porph.

[sketch] A B

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1835 St. Jago — Mendoza (6)

breccia, dipping at 40° to the SW, that is directly towards the granite, to the Eastward Westward a mass of strata on the top of the very same slope of the hill may be seen dipping from the range of granite (B). — The line of porph. brecc. which dips SW is also remarkable from the strata gradually bending up, as at Cauquenas, till they become nearly vertical. — It is in such irregular lines, that the valleys, which approach the central Cordilleras in zig zag steps, traverse find & enlarge openings. — The porph. breccia shows here in many parts its mechanico-sedimentary origin very plainly: in a smooth surface, lines of deposition of different shades of coarseness of the component particles are very manifest, although all now blended into a compact stone. Amongst some of the upper strata there were white & other coloured rocks, the first appearance of a great formation which will be soon described. — I also saw a considerable quantity of a species of porphyry, with abundant oblong rectangular cyst. of feldspar (like 2757) with or without the green chloritic balls. — I believe this rock is always either in an injected mass or has flowed as a subaqueous lava. — Travelling up the valley the presence, as in the last instance, of granitic rocks was foretold by fragments, extending some miles down the valley. To the South, from the Cuesta de los Indios

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1835 St. Jago — Mendoza (7)

there is a view of a most lofty group of peaks. We had not before seen any mountain of this form, we found it to be owing to remarkable disturbance & the strata being inclined from 70° to 80° degrees or even vertical. — No 1 is a view (therefore only inclined section) of part of this mountain.

It represents an enormous mass. The granite is cased in the lower part by the Porph. Breccia; its upper junction is beautifully distinct, & strongly undulating, small veins are branch upwards. — The superincumbent Porph. Breccia in its lower parts is perfectly crystalline & without a trace of stratification; its dark color is strongly contrasted with the whole granite. — Amongst the huge fragments as large as houses, which lie at the base of the hill. I found some which must have formed part of the junction, angular pieces of porphyry were netted together in a most curious manner by veins of granite.

perhaps the appearance will be better described, by the interstices of a shattered mass being filled up with a formerly liquid substance. — In the upper parts or rough peaks the stratification is very evident & highly inclined, on the right

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473A

[sketch] N^o. 1

(A)
Porph: Breccia
(([andesite] granite))
2000 ft
granite & syenitic greenstone

Porph. Breccia

Porph. Breccia
no stratification
granite

2. Ditto seen from the NW (West end of valle del Yeso)

1 Mountain seen from the S. West (near Cuesta del Indio)

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1835 St. Jago — Mendoza (8)

hand, the direction is different & dip much less. at some distance further on, there is another view No 2 of part of same great mountain.

Here the granite & syenitic greenstone is seen forming as it were a mountain at least 2000 ft high within another mountain. In the first view the granite appears as a great vein between the porph. breccia; here it is clearly seen to form part of a great dome, parts being cased & capped with that formation. —

The peak (A) is at some distance back & many thousand feet high. The strata of all the peaks have a dip from 70 to 80°, & generally to the West (b) (b); this direction does not appear to be immediately consequent on this individual mass of granite but owing to some general line of disturbances

I have called all these stratified rock porph. breccia, they are in fact th such together with the overlying gypseous sandstones, presently to be described. —

N^o 2 is seen from a very remarkable basin called Valle del Yeso, about a mile wide several long & level & it has manifestly been a lake. On the mountain side opposite to this section, there is perhaps one little pap of granite (c), but the

473 verso

(a) I mean to say, that there is a general & violent line of disturbance, running about N & S doubtless of which this mountain of granite forms a part. — yet that the stratification has been chiefly affected by the general line. —

(b) Frequently they are bent, the peaks alone consisting of the most inclined, whilst a little lower down the curve is less. —

(c) From which anyhow veins are sent off & therefore must be an injected rock of some kind. —

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1835 St. Jago — Mendoza (9)

[sketch] N^o 3
(W) granite (E)

N^o 3 far greater part is the stratified rock, shattered & traversed by green dykes. On the W side the strata all dip at about 50 to W., in the centre is the supposed pap of granite, but the section is here much obscured. — On the East side the strata yet dip to the W., but at a much higher angle in places nearly vertical generally about 80°. — the tops are bent over to the East. I am inclined to suspect, that this is an anticlinal point, but & that the strata have been pushed beyond a vertical position. — This section N^o 3 bears N by W from that of N^o 2, the nearly vertical strata, which forms the jagged peak (A) in the latter, appear to be of the same nature & belong to same line of disturbance. —

The Valle del Yeso is elevated several thousand. feet above the plains at the foot of the Cordilleras, from this cause we have first reach those strata which overlie the porphyritic breccia.

On the South side of the Basin, there is a greater part of a whole mountain side composed of gypsum. — the gypsum either occurs in very soft impure & aluminous 262! (a), with mamillated structure, or more

474 verso

(a) This is not considered sufficiently pure to be quarried; sorts such as (2620) are much used in the manufacturing of spirits from the grape. —

475A

N^o 4

(W) F porph: breccia
G
A gypsum 300-400 ft
B green colored stone
C Broken edge [illeg]
C Sandstone
heavy [purple end]
D 1000 ft gypsum
Gypsum (vapour)
E sandstone
F like C
(E)

NE end of the Valle del Yeso

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1835 St. Jago — Mendoza (10)

commonly snow white 2620, with lines of small blackish crystals of carb of lime. & obscurely, thinly stratified. This substance is remarkable by containing numerous large (ie several feet in diameter) concretions of a not very regular shape of a blackish grey hard coarse crystall (gypsum??) ((anhydrite)). 2623 — This is These are sometimes white. — The surface is rough (& marked with band, like those of water deposition) by which it is intimately united with the surrounding soft gypsum. — Small hillocks also project beyond the surface made of beautiful white compact crystall: gypsum (?). 2622 — I saw one little piece of

The external surface of these paps is cracked & mamillated precisely like the outside of light bread, which has risen much during the fermentation. — I saw one little piece of transparent selenite: possibly it may be some subsequent infiltration. — Over the whole side of mountain, there are irregular masses of a stone, which is in quantity nearly equal to the gypsum: its junction was distinct, but the connection very obscure, (to my shame ...)

I at the time imagined these to be sandstone concretions of the same structure as those of the hard gypsum (?). — I find now they are all finely granular crystallized feldspathic rocks, either ferruginous with yellow powder. (probably decomposed pyrites) 2625 & a

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1835 St. Jago — Mendoza (11)

little green mineral 2624. or more commonly greenish grey & bright green from having much epidote. 2626 — It may be remembered. that nearly opposite (N^o 3) there is an extraordinary interlacement of green dykes: I am inclined to imagine that these masses are dykes enlarged & irregular, from the softness of the gypsum. — The appearance of the gypsum induced me to believe that it had undergone the action of heat. if this feldspathic stone is of igneous origin, it must have suffered much from that cause. —

From the alluvial action it is difficult to observe the stratification; the inclination is great, — in the distance however the white strata a little to the East of Peak (A. N^o 2) are seen vertical & parallel to the red beds, from the extreme disturbance I could not be certain about its position [platform]. — Another fact perplexed me far more; the main body of the gypsum vastly covers uncomfortably a point two or three hundred ft. high of a compact, blackish, conch: fract: limestone & a black calcareous, carbonaceous clay-slate 2627 2628. These are stratified vertically, & certainly are not part of the gypsum formation, we must suppose that either a fault has brought up an irregular lump of the

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1835 St. Jago — Mendoza (12)

lower strata, or that it is part of some distinct formation anterior to the porph. breccia. This last supposition depends on subsequent proofs, that the gypseous & porph. breccia formations are conformable in all other parts. — I know not which to incline to; I must confess that I have no where else seen such varieties, yet & on the other hand I have never met any unconformable or distinct strata beneath the porph. breccia. —

A little way higher up on North side we have an opportunity of ascertaining some of the beds which are associated with the gypsum. — Section N^o 4.

We have in the centre a great thickness of a compact heavy, purplish-red fine grained sandstone (CC) 2608. — I may remark that in no way part lower down can a rock with of so little chemical crystalline structure be found. The only sort I recollect is some jaspery & claystone substance, found in the Bell mountain, & which would appear to be the semi-altered basis of the porphyries. — The sandstone is covered by a grand bed of gypsum, when I say 1000 ft thick I believe I understate the number. — Some of the gypsum has a saccharine fracture 2609 & black water lines, contains a little carb of lime chiefly in these lines; generally has concretions &c. precisely as described before. — The gypsum is again covered by red (E) stratified rocks, similar to (C)

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1835 St. Jago — Mendoza (13)

Beneath the central bed (C) is a stratum of some (B) green colored rock, which I had not time to examine.

Again (a) beneath or to the West of this we have gypsum (A) to the thickness of 3 or 400 ft, of a similar nature with (D). — The stratification on the right hand may be seen to be only gently inclined, from this, to the left it steadily increases its angle. till at (B) it is 70° & in the gypsum (A) nearly vertical. —

To the left of the latter, the section is concealed by rubbish, but a few hundred yards further on there is true porphyritic breccia, dipping at ∠ 45° to the West. — Hence at (G) we have an anticlinal line & a great fault. — Right on the opposite side of valley, in the line of upheaval, a mass of white gypsum, corresponding to A. No is supported on each side by conformable vertical red stratified rocks: Those to the West, which probably correspond to (F) are united to a hill of porphyry, where the stratification appear obliterated much in the same manner as in N^o 1. —

From this NE point of the Basin of "Yeso" to the foot of the grand ridge of the Paquenes, which divides the waters is no great distance. The formations are similar to what have & will be described, the

478 verso

Basin near the mine of gypsum

(a) I also saw at the mine, thin layers of some green substance, frequently curved, alternate repeatedly with the white gypsum. — By referring to Copiapò, the probable nature of this green rock will be discussed.

It is there a hard compact, pale green, fine grained (either calcareous or not) sedimentary rock: particles almost blended together like (2988 & 2990). It also occurs at Coquimbo.

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1835 St. Jago — Mendoza (14)

stratification exceedingly disturbed. — The steep ridge of this first Cordillera of Paquenes is formed by strata dipping to the East at angle 30°-45°. — but, as is generally the case, they bend up to a much higher inclination in the groups of peaks on each side of the road. — The lowest strata are seen to be a red stratified rock, & I believe (from fragments) a sandstone in part conglomerate; above these we meet a grey stratified limestone containing abundant traces of organic remains, perfect Gryphites are very numerous; with them there was a bivalve & a few fragments of ammonites. 2615: 15: 17 2618 2619 — The best specimen of the latter was so large I could not carry it, being thicker than my arm on although only a small segment of the curve. — Higher up this stone passes into a jet black limestone. I call it limestone, but it is manifestly very impure & perhaps better deserves the name of calcareous clay slate. — This rock is thinly stratified, layers of a more compact, conch kind are divided by thinner laminae of a brittle stone. with conch. fracture 2610.

In the former variety there are many small bivalve imperfect forms of bivalve shells, whi such as Terebratulae, only one univalve, but several bits of ammonites 2611: 12; 13: 14. This rock although ink black in its fresh fracture, externally always weathers into a very pale color. —

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1835 St. Jago — Mendoza (15)

In the middle of this limestone, there is a stratum perhaps 200 to 400 ft thick of white gypsum, & is strictly conformable; in its nature resembles that of section No 3. equally contains the hard white concretions & some green colored stone. —

The total thickness of the grey. & black limestone with the gypsum is very great. it is difficult to estimate distances, but I should say 4000 3000 ft: would not be too much. — These beds are capped with some red sandstone, the absolute superposition is not seen, but I have no doubt for the subject, shall soon show reasons for thinking so. — Such then is the constitution of the Atlantic & Pacifick oceans. the its height is probably about 13000 ft (a), the lowest gap being above the lowest limit of perpetual snow & the peaks are enveloped in it. — The elevation in this latitude, according to Humboldt (Caldcleugh's travels)¹ of the line of perpetual snow is 12800 ft. — The direction of the crest is about NNW & SSE (& therefore the dip ESE). I may remark that generally the lines of upheaval which I have called N & S. have some westing in the northern point.

¹ Caldcleugh 1825 1: 310.

480 verso

(a) although there is no deficiency of such examples this is a good instance of the height at which marine remains are found. To me the fact was very striking, because I had formed an idea, that the higher parts of the Andes were all volcanic. —

481A

[sketch] N^o 6
W E
(a) (B) c z D

East foot of pass of Paquenos

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1835 St. Jago — Mendoza (16)

To the West of the ridge the strata are much broken up: near to the grass, there is a small mass of the black limestone with a West or opposite dip. — V. section N^o 5. — a little distance To the North of this I saw not less than four small ravines formed in the troughs of the anticlinal & synclinal strata.

[sketch] W Paquenes E

I have not represented the manner in which the strata curve upwards in the little peaks on each hand. —

To the Eastward to of the main line, the stratification & superposition will best understood by looking at Section N^o 5. — The strata of black limestone, having a mean inclination of about 40° gradually are arched. as they descend to the side of a narrow ravine. On the opposite side of this we have nearly vertical. yet yet with small Easterly dip beds of a bright red sandstone. — These have the form, as if continued they would lap over the limestone. — This section is on the North side of the East-West line of road in the valley between the two Cordilleras. — on the South side & in front the structure is represented in Sect N^o 6. —

Here the limestone is as gradually inclined as less to the E. till meeting with a line of fault, it dips to same point at about 70°. — There can

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1835 St. Jago — Mendoza (17)

be little doubt, that the same force which curved the strata in N^o 5, in this case moreover obliquely lowered the part (a) when say that the line in which were formerly continuous with (B), now meet them at a larger angle. In the part (B) the curvature may be seen to increase till it becomes vertical; — Here the red strata of sandstones are seen in conjunction & conformable, there can therefore be little doubt, that originally they were superimposed on the limestone. To return to (Sect 5), beyond (c) we have valley & heap of rubbish (z); then a hill of the same red rock (D). the strata however instead of dipping at very high angle Easterly are inclined to the W at about 45°. — In sect. 6 a similar structure may be seen only there the hill D is more separate & further off. — The strata of (D) rest on the mass from (E to H.)

These are black limestone, similar to the Paquenes ridge & likewise contain a similar bed of gypsum (F). (a) V (a). I believe this limestone rests on red sandstone, but am not sure. — All these latter strata dip at about 45° to the W. — Mere inspection will show that, C & D are parts of one the same set of strata

482 verso

(a) It must be particularly remarked that the the inclination of the strata in the limestone (W) be is greater (about 60°) than that (E) above the gypsum. —

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1835 St. Jago — Mendoza (18)

(A & E) the gypsum (F & F) & (H & H). The principal line of upheaval has tilted the strata of Paquenes to the East. To the W the lesser elevation may perhaps be explained by the more fractured state of the strata owing to the lesser other parallel & approximate lines of disturbance, as seen in Page 16. —

On the synclinal line (z.) I think it probably is owing to subsidence, rather than to being the neutral line between two upheavals; such was my opinion, when beholding the stratification. — As far as we have yet seen of the gypsum (& we shall see no more of it in this pass) it is interstratified with red sandstone (a), or with black aluminous limestone.

I am not certain about the relation of the sections 4 & 5. i.e. of those strata which contain the gypsum.

We shall hereafter see that this formation is subject to great variation & that there is scarcely any limit to the thickening & thinning out of the gypsum. This being the case different as the rocks are I should not be surprised if the the black limestone & red sandstone replace each other; indeed this is my opinion. If otherwise the superposition must be nearly as below. —

483 verso

(a) The red sandstone is seen in both places; the converse but it must not be supposed that the black limestone is only found one. — Not far from the gypsum quarry there were some laminated rocks of a pale nature external color, which possibly might have been the limestone on compact calcareous clay-slates. —

Ridge of Paquenes
{ Red sandstone
{ Black limestone with ammonite
{ Gypsum
{ Black limestone with do
{ Brown. do with gryphites
{ Red sandstone

NE Valle del Yeso
{ Red sandstone
{ Gypsum. grand bed 1000
{ Red sandstone
{ Gypsum
{ ? ?

Chiefly found in lower & external parts of the Cordilleras
{ Porph. Breccia being altered conglomerates, sometimes containing limestone. jaspery beds & probably subaqueous porph. lava
{ Granite & syenite greenish
{ cause of metamorphic action

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1835 St. Jago — Mendoza (19)

The connection of the gypseous & porph: breccia formations cannot here be discovered. — Indeed in front of the section N^o 4 Page 13 there is weak. presumption of their conformability. — In the Uspallata pass, we shall see conglomerates partially altered supporting the gypseous sandstones & in the North of Chili the whole grand masses can be seen in uniform superposition. —

We have now reached in our passage about nearly 1/3 1/2 of the distance of the elevated (perhaps rose at 9000 ft 7500 when rise crosses) land V P.2 (a), which divides the two main ridges of the Paquenes & Portillo. We left off in the description of Sect. 5 at the limestone beds (H) dipping to the West at about 60° & reclining apparently I believe on red sandstone. —

All the formations to the East of this will be found to belong to a distinct & more recent period. — We first enter on a grand mass of stratified red coarse conglomerate. (V Sect 5 & 7). For 2 or 3 miles we had this on either hand. — It is divided into irregular hills & parts rise to great heights, although not equal to the Paquenes range. —

The last limestone beds dipped W to about 60°, the first of the conglomerate about 20° to same point

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1835 St. Jago — Mendoza (20)

This low inclination is continued for some way & is succeeded by one of 45°. — The strata with this dip are of immense thickness. — I cannot should suppose the elevation has been [sketch] of this form, where a succession of parallel faults prevents the escarpement from being of an excessive height. — The thickness however must be at least from 1500 to 2000 ft. When having well entered this formation & looking westward, the deception is perfect, but the beds dip beneath the limestone. — I at first thought so, & was much surprised at finding, in so low a position, a conglomerate 2629, cemented by calcareous matter & so little altered by any metamorphic action. — Its appearance in many parts was that of a cliff of shingle by a river course, but in truth the rock is hard & all small pebbles can be broken more easily than separated.

With such doubts in my mind I was pleased to find the pebbles consisting of sandstone quartz, porphyries, sandstones which accompanies the gypsum & lastly abundantly the black limestone of Paquenes, with its small bivalve

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1835 St. Jago — Mendoza (21)

fossils. The modern origin of these strata are is manifest. — It will be seen they lie conformably over beds, which have been tilted, metamorphosed, injected by grand granite ridge of the Portillo. — The Paquenes line must have existed for a long period as sm islands above the sea, to allow of sufficient degradation to accumulate such a mass of waterworn pebbles. It is probable that the exterior masses of limestone (H) of that period had a less dip than those nearer to the axis. —

When the granite burst up at the Portillo the conglomerate were tilted to the West, but the inclination of the parts, furthest removed, would be less affected: hence the dip of 20°. — This however added to the previous inclination of limestone would give a dip now greater than the parts (E), near to the synclinal line (z).

In the centre of this recent formation, there rises a lofty mountain of porphyry (lilac coloured with black crystals). 2630 — in this there are no traces of stratification or mechanical origin, a fact

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1835 St. Jago — Mendoza (22)

which in this pass certainly is not very common. —

To the South of the road, there is a mountain with strata dipping at small angles to East or towards the granite chain (b), which from color &c &c appears like porph. breccia.

These hills are parts of a formation, which on this side of the Cordilleras is quite overwhelmed. The conglomerate rests on some curiously colored soft strata, I had not time to examine them. — I think it not improbable that they correspond to a large formation which will be described at Uspallata. — These beds rest on a large sandstone formation, which in its lower parts touches the granite. — The sandstone is fine grained hard, siliceous, micaceous, grey colored 2631, is divided into very even & numerous laminae (a); is associated with a little hard black clay-slate; is traversed by several dykes of decomposing porphyritic rock. — The sandstone near to the granite is converted into a stratified granular quartz rock 2632, containing specks of black mica: the junction of the two rocks is beautifully distinct; the granite being of a bright red color, the its large, numerous & contorted dykes are, as it were, painted against amongst

487 verso

(a) As there is no corresponding formation in the western range & as this is conformable to the conglomerate, I believe, (but cannot know) that the sandstone also is of a age posterior to the partial elevation the Paquenes ridge (b) The angle is now small, not more than 20°, perhaps the tilt from the granite has only lessened a former high easterly dip. — from original inclination

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1835 St. Jago — Mendoza (23)

Sect: 7

the strata of grey sandstone (b). Shortly after beginning to ascend the Portillo range, the road winds between lofty & bare peaks of granite.

The granite in nearly all this side is composed of large red crystals of feldspar Protogine, with large do of quartz, generally affecting a linear arrangement, with these sometimes is associated a little talc chlorite, hornblende or mica. 2633 —

I also saw one distinct mass of granite, porphyritic with large cryst of white feldspar. — At the very "Portillo", or gateway over the ridge, the rock is either fine grained granite or syenitic greenstone. it contains numerous vein-like masses of a flesh-red or green euritic (or feldspathic? stone. 2624 2636 (a). —

At the time, was was falling a frozen mist, hence I could not examine beyond the ver road. the whole of which is granitic

M^r Caldcleugh states that the magnesian limestone is found there.¹ — I certainly saw a patch at a distance of some colored stratified rocks like the non-descript at the base. —

High up on the East slope, I found a few fragments of quartz & mica slate. — A good way

¹ Caldcleugh 1825 1: 309.

488 verso

(a) In the veins there is also some decomposed. wacke. —

(b) The nature of this stratified quartz rock reminds me of that of the Falkland Is.^ds

[sketch] sandstone protogine

There was a still stronger resemblance in one spot in the stratification: at the base of the inclined mass which rests on the granite there was a small dome, with the strata regularly curved. — it can not be doubted, that there is a point of the injected rock beneath this arch. —

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1835 St. Jago — Mendoza (24)

(Sect.ⁿ 8) lower down, at Los Manantiales, where vegetation commences there is much fine grained true granite & syenite. In front of this & universally lower down the valley the red protogine prevails.

In some spots I observed that this coarsely crystalline rock passed into a compact substance resembling the veins injected = the "Portillo". white granite. Observing this & the distinctness of the hills composed of the two kind. I suspect their ages are distinct different, the protogine being posterior. (a) —

At Mal Paso I first observed the protogine capped by a horizontal mass of mica slate. The junction from the same cause as on the East West slope was very distinct; amongst some very large fragments I found one where I could examine it closely. The laminae of the slate at right ∠ to the line of junction; it was penetrated by more many small veins [sketch] entered between of the granite & a few pieces, with their perfect character were enclosed within the solid unstratified mass. — I noticed this formation in other spots, till at Los Arenales, the mountains are composed of it. — the mica slate consists of a micaceous mineral with quartz 2636, the latter generally

489 verso

(a) Even if the Protogine is not posterior to the white granite. it is so to the upheaval (owing to the granite & syenitic greenstone in the West Cordilleras) of the Paquenes chain. — Therefore its name of Proto is here very inapplicable. —

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1835 St. Jago — Mendoza (25)

in thick contorted layers. 2637 — The micaceous mineral in some many places passes into one a glossy clay slate, which in one spot occurred without quartzose layers. 2638

When the mica slate was most abundant, the cleavage dipped at high angle easterly. — This whole formation strongly resembles the great ones of the coast of Concepcion. Chiloe & Chonos Archipelago.

As we have seen sandstone converted into quartz rock, so may this mica slate be some mechanico-sedimentary deposition of the same age; but I rather incline to believe it part of the great flooring, which extends over all this part of S.America. —

Its (metamorphic) origin may with more probability attributed to the whole granite. —# I have seldom seen a more interesting view than looking up to the Cordilleras from los Arenales. — The contrast of the colors of the black micaceous schist & red Protogine is so strongly marked, that it can be perceived even from the Pampas (a); the red however is far predominant. — The vertical planes of fissure cause the hills of Protogine to assume the wildest forms; the sharpest peaks are sometimes just capped with the black rock, in a other times, mere

490 verso

(a) [sketch]
mica slate

(b) I must observe that judging from the appearance this formation of Protogine, cased by mica slate extends for many miles to the northward; & several to the South. [& e] as far as I could see

steep eastern flanks

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1835 St. Jago — Mendoza (26)

patches adhaere to the sides. I observed between these a remarkable correspondence on the opposite sides of the deep vallies which have been excavated in the Protogine. On the other hand, in the hills of mica slate, occasionally a red point will project. — More inspection would convinced convince a beholder that once all the unstratified rocks had been covered or cased by the slate. —

At Los Arenales above the side of a hill of mica slate, about 1200 ft high, there is a field of Lava beds, which present a perpendicular cliff to the valley. — The mica slate is here also remarkable by a grand system of dykes. I counted eight of nearly same constitution, being a pale brown feldspathic base, with cysti of feldspar & quartz 2639; generally the base is not porphyritic with such large crystals.

[marginal] Like Chonos [as well as band]

Most of the dykes are from 20-30 ft wide, they are so exactly parallel to the cleavage, that it was only by closely examining the junction, which was sinuous & showed signs of violence, that I could tell their origin. — This was the more necessary, as in one spot, they equalled the mica slate in quantity. — Higher up the hill I found some of them had broken through the

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1835 St. Jago — Mendoza (27)

trammels of the laminae. There was also a black an irregular dyke of blackish crystal nature 2640, & another near to the junction of the mica slate & granite of a chesnut brown ferruginous kind. 2641 —

The lava field, above mentioned is from 300 to 400 ft thick, consists of many streams. To the W & S is bounded by the hills of mica sl[ate] & granite; to the E slopes down, in an irregular outline to the edge of the Pampas; I do not know whether the whole inclination can be attributed to the slope of the ground over which the lava flowed, but rather suspect it is helped by a tilt from the granitic axis. — To the North it is intersected by the grand valley, in which the road lies, on the opposite side however portions of cliff are to be seen at the corresponding height.

I could not find (detritus) the arterial junction of the lava & mica slate; the lowest bed, consist of a laminated grey harsh stone 2642 with much olivine; the plates are frequently curved, so as to become vertical. (a) — This is covered by 25 ft of cemented pieces of a compact red scoria (2645) 2645 the cement is whitish & firm, contained one pebble of basalt it appears an aqueous deposit.

492 verso

(a) This band is not very regular either in its thickness or straightness. —

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1835 St. Jago — Mendoza (28)

Above this there are some streams of lava, which is chiefly minutely vesicular (in planes) greystone with olivine 2643, black mica, & glassy feldspar. 2644 — Amongst the fragments there were blacker kinds with (zeolite?). —

These beds were covered by the cemented breccia of red scoria as below. — And in this, there was a thick mass of alluvium chiefly angular, some rounded pebbles, in places cemented by a white calcareous friable matter, like mortar.

Sect 8

From the appearance of this & the scoriae, I feel no doubt that there have been subaqueous lavas in a shallow sea. — There is no trace appearance of the volcano; probably the whole has been removed, that it stood on the NW border between the hills of Protogine & the margin of the existing field. — Since it the lava flowed, the main valley probably (perhaps about 1500 ft deep) has been excavated. Within the valley, some way lower down, (near the Guard house) there is a considerable formation of white tufa. — The stone is of very little specific gravity, but firm, is used as a filtering stone; on consists of bits of pumice scales of mica, broken crystals, & small fragments of hard rocks all cemented together, in white volcanic ashes.

493 verso

(a) I saw lower down the valley a facade of columns

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1835 St. Jago — Mendoza (29)

The surface of the tufa is much water worn it only remains as rounded hillocks, or as a line of cliffs within those of the lava. — This was deposited when the valley was partially excavated (hence at this time the volcano must have been activated active), & but subsequently the mica slate has been deeply cut beneath its level. — Everything induces me to believe that the tufa was deposited, when this broard valley was a bay open to the sea, covering the Pampas. —

Within the tufa, I saw a pap of white granite covered by mica slate, which shows the continuity of such formations beneath the lava. — The field of lava follows its irregular course downwards & on the edge of the Pampas abuts against a low range of porphyritic rock. — is here perhaps 200 ft thick.

At present a broard, plain like valley has cut through both lava & the chain of hills, but & clearly has all been formed since the period of the igneous stream. —

I may here observe that so far from the Cordilleras being composed of volcanic rock in the ordinary acceptation of the word, with the exception of this one place, I have nowhere seen even

494 verso

Humboldt Vol VI p. 582 talks of amygdaloid works on border of Missouri plains & ford of Venezuela range¹

¹ Humboldt 1819-1829, vol. 6, II, p. 582.

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1835 St. Jago — Mendoza (30)

a detached fragment of any cellular rock, or otherwise manifestly melted without great pressure.

Yet the road passes in view of the great mass of snow clad mountains, called Tupungato, out of one of which smoke has been seen to proceed.

At Moreover the volcano of Maypo, taking its name from the valley, by which we ascended, although hidden from the view is not far distant.

After leaving descending from the granitic central chain, all the hills are of very inferior height. Beyond the limit of the lava & upon the sloping talus which blends into the level Pampas, they are more mole-hills from 3 to 600 ft. — These extreme hills consist of several irregular little water-worn chains, which run obliquely; here the direction is NW by W & SE by E. — They are separated by a plain of shingle, where the pebbles are whitewashed as in Patagonia. — All are porphyritic; then the western line, is distinctly brecciated (a) & has an obscure stratification dipping SW 70°. —

The others are true porphyries, one common variety consisted almost of a mass of large brick red cryst.^s of feldspar. — others were whitish feldspathic porphyries. — Reviewing the sections of the Cordilleras; represented in papers. (a, b, c, d e ).

495 verso

(a) The rock is colored bright red & purple contains some grains of quartz

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1835 St. Jago — Mendoza (31)

I must distinctly remark they pretend to little or no accuracy: They are merely drawn to give a general idea of the structure. — It will be sufficient to state, that I only once, (but not quickly) crossed this pass; Day by Day I made partial sketches & here they are all united. The part to the east of the Paquenes (inclusive) is much more exact than to the West. — The valleys in the first part bend to SSE, SE & then up again to NE. hence the difficulty of forming a guess of the structure in a straight line; from the Paquenes however the road runs nearly direct East to the Pampas. — Of course the section was sometimes seen to the South, sometimes to the North of the valley. — The distinctness of stratification, bright color of the rocks, clearness of the atmosphere, absence of vegetation, renders much more possible here than in most mountains. —

The vertical height is exaggerated. from two to three times beyond the true proportion. —

In Sect. 8. I have attempted to draw the scale.

496 verso

Passes [Portillo] 14,365

Paquenes 12,270

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1835 St. Jago — Mendoza (32)

From the heights of the cities of St Jago & Mendoza above the sea I imagine the valley of Maypo., where section commences, to be about 2500 ft & the edge of Pampas 3500 ft. — From the maps. (longitudes of Mendoza & St Jago) the breadth must be about 60 nautical miles. — From Barom. observ in the Uspallata pass. & height of line of perpetual snow, the passes must be about 12000. & the neighbouring peaks 14000. 16000 ft — I have joined the extremities of the base with the two lines of peaks. — it must not be supposed that the mountains gradually decrease in this ratio, for on the contrary, the Cordilleras close to the limits rise to a very great height. — The western limit is more arbitrary than the Eastern one of the Pampas, it is however well marked by the plains which extend to the South St Jago. — I have also drawn the volcano of Aconcagua, the highest summit in central Chili. 23000 ft — The sea coast is about 60 miles from the western limit of i section. —

The most striking fact, on first inspection is the dissimilarity of the formation on either the two

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1835 St. Jago — Mendoza (33)

sides of the Paquenes. — The posterior elevation of the Portillo chain, I think satisfactorily explains this. Such strata would be torn & broken & exposed to vi excessive degradation (to in open sea on Pampas) even if the strata parts were not melted to form the very red Protogine. — Remnants of the porphyry yet remain, & probably still more in the intermediate district concealed by the modern conglomerate. — I am inclined to suspect, that when this conglomerate was accumulating, there was a low ridge, where the Portillo now stands composed of white granite & mica slate. —

Hence the white quartz pebbles in the conglom: —

It may be likewise a matter of surprise, that the gypseous formation does not extend further West. — I must however state that I do not know its exact furthest limit. — I believe from what i have see this formation is always very limited, as if deposited in basins; likewise, when the central parts were elevated into dry land, the lateral might during any period be exposed to degradation from the action of the sea. — In the more northern parts of

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1835 St. Jago — Mendoza (34)

Chili, however this formation does extend far from the main Cordilleras. — In the western part of the section, three sets of granitic paps, will be seen as the axises of elevation; there can be little doubt from their position & mineralogical resemblance, but what they are united beneath the ground. — the same may be said of the Portillo range. — Mere inspection will show the probability of a granitic axis beneath Paquenes & if so all the stratified rocks rest on a solid doom of this substance; its nature however is rather different on the Eastern & Western sides. —

In the Portillo, the sub-aqueous lavas must have flowed from a volcano in the granite & mica slate rocks: the volcano of Maypo is not very far from the group of granitic paps at the junctions of the Rivers Yeso & Volcan. The opening conduit must pass through from the inferior crystalline rocks, through the porphyritic breccia. — Volcano of Tupungato is seated in the intermediate district, but nearer to the Paquenes & to the North of the pass. —

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1835 St. Jago — Mendoza (35)

the conjecture frequently occurred to me; I have already stated the very general occurrence of a white syenitic greenstone & granite in the Cordilleras: the general trachytic nature of the lavas, quantities of white pumice, tufa &c is well known, as characteristic of the volcanos,

Does not the latter fact depend on the former. How trifling is the difference. excepting in compactness between the syenitic greenstone & common trachyte & greystone!

With respect to the stratification, all the most lofty & central lines of elevation run nearly N & S., generally perhaps with a little to the West of the North point.

I believe on the West side of section the number of these lines is not sufficient. — The stratification in the central parts is much more disturbed & the inclination higher, than in the outer parts; hence in the latter the mountains are grand massive ridges in the former wild peaks. — Very commonly the strata, gradually curve upwards & where vertical, form a group of these peaks. — I saw more of the highest points without a high dip of stratification

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501

1835 St. Jago — Mendoza (36)

In the North of Chili the Andes consist of three & four main ridges; inspection of will show how easily this might happen here, by a little more injected granite in any of the parallel lines such as those in the Valle del Yeso. —

I must particularly mention, that the smaller chains on the outskirts on both sides of the Cordilleras are very commonly oblique to the central ones. — Hence the spurs which indent the bordering plains. It may be well doubted whether these are of the same age as the grand central ones. —

I believe this section give a good general idea of the structure of these great mountains: it will be seen in the Uspallata pass how far the main features are repeated. — I by no means expect, that the same lines of elevation are continued without interruption, or the appearance of the its granitic axis; neither will the same kind rock or exact superposition be found universal. —

The two main points of interest in the geology of this pass; are, 1^st the palpable igneous & injected origin of the syenitic greenstone, granite & protogine which forms the various axises & the easy (a) [continued on 501v]

501 verso

(a) explanation, which the supposed continuity of these substances beneath the ground, offers for the extreme metamorphic action in the whole country west of the Paquenes: 2^d The elevation, & formation by igneous & sedimentary causes, of the Portillo range subsequently to the existence of the Paquenes as dry land —, the former a chain as high, (if not higher) & parallel to the previously latter & older one.

This is contrary to M. Elie de Beaumonts theory, & what is more important, shows that the Cordilleras, one of the best defined ranges on the surface of the globe, has not risen by one blow, but by accession after considerable intervals. — It may also be remembered that even in the Portillo there are two distinct granites. —

¹ Élie de Beaumont 1829-1830. "A reference to Elie de Beaumont's theory of paroxysmal elevations which explicitly suggested a single episode of elevation for the Andes and also that structures in mountain ranges always tend to be parallel. Darwin had, of course, collected ample evidence that the Andes did not all appear at once, such as the several 'axes of elevation' each with its own granite backbone, with long intervals between the elevations. The Puquenes first, then the Portillo which itself had two granite axes. Also there are the outer ranges oblique to the main ones, so they were not parallel and not the same age. In the first para of p. 501 Darwin seems to speculate that the Santiago-Mendoza ranges might look like northern Chile if more granite is intruded." Gordon Chancellor.