RECORD: Darwin, C. R. Geological diary: St Jago. (1-2.1832) CUL-DAR32.21-36 Transcribed by Guido Chiesura, edited by John van Wyhe. (Darwin Online, http://darwin-online.org.uk/)
REVISION HISTORY: Transcribed from the Darwin Online images by Guido Chiesura, corrections by Gordon Chancellor, corrected and edited against the manuscript by Kees Rookmaaker 6.2009, corrections and notes by John van Wyhe, corrections by Thalia Grant 7.2009. RN6
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Reproduced with the permission of the Syndics of Cambridge University Library and William Huxley Darwin.
See the introduction to the Geological Diary by Gordon Chancellor.
1832 Jan 17 to Feb 8.th St. Jago.
The island of St. Jago, as seen from the North presents a succession of varied outlines; Towards the coast the general formation is steps of Table land. — These are traversed by
by broard & abrupt sided valleys. often leaving patches in the shape of truncate conical hills. —
The contrast is not so striking, as here mentioned.
Rising out of these plains are seen two classes of hills. the one of a pyramidal regular figure. the other more abrupt & jagged & flat topped. evidently bespeaking a different formation. — In centre of island a chain of more lofty & bolder shaped mountains extends itself. —
Mt. S. Antonio height 4400 feet.
The whole country has a desolate wild appearance & it is impossible for an instant to view it without attributing the formation to volcanic origin: The general tint is a reddish brown which, excepting in some of the interior valleys, is relieved by no vegetation. — One remains in doubt whether
most it is easier to account for this utter sterility by the heat of a Tropical sun or the agency of the former volcanic fires. — The black & precipitous cliffs that surround the Eastern side of the harbor of Port Praya are diss traversed by a white band, which stretches away for some many miles in an horizontal direction. This line which is very striking proves to be the upraised beach of a former coast: as most of the rocks in the island bear a relation
The contrast is not so striking, as here mentioned.] added pencil.
& flat topped.] added pencil.
[Notes to page 22 recto]
bottom of the beds deposited near the coast, from shells being same as now on the beach
(a) I take Lava in a large sense (Dic. Class.)1 as any rock that has clearly flown in a stream
(b) And that after being thus covered the whole mass was by some enormous force raised to its present position:
(c) These shells have lost their animal matter, that is they do not when heated alter or emit an animal smell
bottom of the...on the beach] added pencil.
1832. Jan: Feb St. Jago
to this period: it has appeared to me a good one to commence my sketch of the formations. —
This white zone which for the future I shall call former
coast bottom (d), rests on highly crystalline rocks & is covered by a field (a) of Lava (b) [small sketch] The section of coast seen in the cliffs varies in thickness from 6 to 25 feet. — the height of its highest superior beds above high water marks likewise is not constant. On the coast East of Praya, the phenomena are most regular; the line here as observed by a Theodolite is truly horizontal. & exactly 45 feet high its thickness being about 20. — [in the left margin to this section: ?] & the superior covering of lava 50 feet thick. In other places, as Quail Island. & Flag Staff hill the former coast dips under the present sea. — The line to the West of Praya is visible near to the Red Hill. about a mile & ½ but to the East. I have examined it for about 7 miles & I think it probable that it extends much further. — The general character of the beach is the an abundance of calcareous matter. — occasionally it is either conglomerate, earthy or sandy & even indurated sandstone. — Every where half the constituent part is the minute fragment of shells & Corallines: interspersed (c) on these are found numerous perfect shells; which I think are certainly the same
bottom (d),] added pencil.
[Notes to page 23 recto]
(1) Balls occur in beds. generally in the upper & central part of the beach. —
(2) Not universally so
(3) These ostreas occur at the very bottom where there are stones there for them to adhere to
If this arrangement of shells is universal one would suppose that the bottom of sea was gradually uplifting during the deposition of the bed, so that at first Oysters could flourish & subsequently Patella were scattered on the more shoal bottom
(4) & the Basalt cellular & abounding with Carb of Lime. —
If this arrangement...Carb of Lime. —] added pencil.
1832. Feb — Jan St. Jago
as now exist on the coast. — Amongst them may be mentioned. Cardium Isocardium Voluta, Conus, Patella, Buccinum, rugged Ostreas. (3) adhering to the stones where they lived, Turritella &c. &c. Coralline: Crabs. Echinus. — The Turritellas are exceedingly numerous in the lower & more earthy beds; the Patellae likewise in the upper: which seems perfectly in accordance to the habits of their genera at the present day. (2) The beach where it comes into contact with the superimcumbent lava is converted into a hard white rock with yellow spots (21) (4) — By far the most extraordinary phenomenon in the former coast (1) is the universal occurrence of numerous balls of amorphous Carb of Lime. —
1836, found also at base of Flat Hill in conglomerate
these mamillary concretions vary in size from a walnut to a small apple & have generally in their centres a small pebble. — (25 & 26 &c) The lime in places is so abundant that every body is incrusted with it. — I have observed in neighbouring hills the tufa with calcareous base, intermingled with large beds of pure lime. —
All the volcanic rocks seem here to be mingled with much calc: matter
it is from
this such sources. I should think, that the lime came in the antient sea. — The former coast is generally separated from the lower crystalline by a thin bed of pebbles. I shall have occasion again to notice this, in order to prove the preexistence of rock
1836, found also at base of Flat Hill in conglomerate] added pencil in margin.
All the volcanic rocks seem here to be mingled with much calc: matter] added pencil in margin.
[Notes to page 24 recto]
(a) Dr. Daubeny1 when mentioning the present state of the temple of Serapis, doubts, the possibility of a surface of country being raised without cracking buildings on it. — I feel sure at St. Jago in some places a town might have been raised without injuring a home. —
Owing to this extreme regularity I at one time felt inclined to think that the sea must have sunk instead of the land raised. — but as this supposes the fall of the whole Atlantic (
& the bottom round St. Jago) it is clearly impossible:
the different height of upheaval prove it is not by subsidence of water.
(b) the fragments are angular & the base of clay of bright colours green red and blue
(c) amygdaloid with oblong cavities
(d) Great specific gravity & strongly affect the needle.
1 Daubeny 1826.
the different height...of water.] added pencil.
1832. Jan — Feb St. Jago
similar to the one now superimposed on the coast. — When viewing from a distance an extent of cliffs. one is struck by the great force
(a) it must have required to have raised fields 2 or 3 miles broard of these rocks at least 50 feet. (a) — (which is supposing the former coast bottom was at the surface of the sea at high water mark !) —
A considerable thickness of the lower crystalline rocks must likewise have been elevated at the same time. — Taking this into consideration it is perfectly astonishing that the force should have acted so uniformly (a) that a spirit level with sights proved the former beach to be as truly level as the present. —
The rocks on which this antient coast rest has been described as highly crystalline generally (d) they appear to be composed of numerous crystals of Augite & some mica embedded in a Feldspatic base.(nos. 52 …. 70) — In some situations, instead of a rock, it is a breccia of same rocks (b) with a clayey base resulting from their partial decomposition. — In one place, under Flag Staff Hill, rounded pebbles of these rocks are mingled with the breccia. — In same situation beneath coast is found a basaltic amygdaloid with minute crystals of Carb of Lime in cavities.(48 …. 51) — The whole of this formation (c) is almost characterized by the
[Notes to page 25 recto]
(a) These dykes often assume a transverse columnar or prismatic form
(b) The crystals probably are mostly olivine: the breccia is angular:
The top of this hill in some respect looks as if it was a piece of old table land obliquely raised by a mass of melted matter from below. —
(c) These augitic rocks for a few miles on each side of Praja is cut up in every direction by these dykes: which solely differ from neighbouring rock in being more compact & crystalline & taking a prismatic form. — Are not these differences explainable by supposing the
mass dykes to have cooled more slowly. — I cannot help conjecturing, that the augitic rocks might be considered these appearances may be explained by considering the augitic rocks originally to have been a crust of over a melted mass — & that when partially cracked from [word deleted] cooling then numberless dykes were injected from the inferior mass. — In this case the walls of the dykes being yet hot. the dykes themselves would cool very slowly. — this likewise explains the side not being altered by the heat of dyke. —
1832 Jan — Feb St. Jago
number of (a) dykes that cut through & interlace even part of it: even through [word deleted] decomposing mass already mentioned. — These dykes existed before the deposition of former coast: they consist of augitic rocks, which are very hard & resist decomposition. — (no. 62, — 146, — 83. Bordering rock, 84) I traced on the coast W of Praya one of these dykes running for some hundred yards in a tortuous direction. In no case could I observe that the walls on sides of these dykes were
not altered in which respect how widely they differ from the greenstone ones. These rocks are hidden inland of the coast (c) by the beds of former beach & covering of lava. —
But I consider the hills in the interior, although differing in these constituent parts to be of the same formation. — These hills which are of an irregular form contain less mica (132. … 137) & the black crystals of augite, but numerous yellowish & reddish ones. In a highest hill N. by E of Praya, the rocks of summit assume the shape of concentric balls abounding in crystals mingled with beds of tufa. (170 … 180) — The tufa is composed of fragments of a cellular rock (b) & together with the crystalline rock in a base of friable white calcareous matter (181 —184). —
Surrounding the bases of the hills of this class there generally occurs numerous Papiform hillocks — The rocks of which
[Notes to page 26 recto]
(a) That in the central part has posteriously been protruded — (?)
(b) 109. Altered rock bordering 108 & 107
(c) As the rock weathers their crystals stand out in relief.
The rock is very hard. — phonolite
Hill covered with angular fragments
The rock is...angular fragments] added pencil.
1832 Jan — Feb St. Jago
these are composed has [word deleted] undergone much decomposition & in consequence they are
covered of yellowish rusty colour (no. 153 & 128 … 130). — They contain mica in small plates, reddish crystals & the base is feldspathic with dendritic marks: fracture largely splintery. — It is difficult to make out these relations to the central mountains. (a) I should think inferior. — They certainly are so to the superficial covering of Lava. —
Amongst these Pap-form hillocks there occurs in 2 or three places N of Praja tapering
& conical cones of a Feldspathic Trachytic rock. Most of them consist of numerous large crystals of vitreous Feldspar (164 … 169 & 106) in a base of the same & mingled with them are a few acicular ones of Hornblende: — (c)
In one situation the bordering rocks seemed to have undergone an alteration (b) & I should think their origin is of a later date than the Pap —form hills, amongst which they rise: — They are previous to the oldest bed of diluvium; & to the upheaving of the former coast & its covering of Lava. — This I judge from finding blocks of this singular looking rock at a higher level than its present situation. —
The only part of the natural section presented by the cliff left undescribed is the covering of rock, which I have called
[Notes to page 27 recto]
(a) As the lava flowed into the sea, may not the cellular state of the upper & lower surface be owing to the action of steam ?
(b) In Quail Island it is of the compact earthy columnar sort (this is merely in comparison: as almost universally small cavities & small crystals may be discovered) & & as is stated is only 15 feet in thickness: it is a curious circumstance that in the most vescicular varieties the upper beds are the most compact when we should suspect there was least pressure. —
Quail Island has been mentioned as probably an island in the former sea & if so it would only be covered by the superior beds of lava & therefore most compact.
(c) Melts into a black enamel & therefore by Dr. Daubenys definition is a Basaltic lava. —
is also magnetic affects the needle.
(b continued) I see Mr. Menard de Groye states that in
volcanic Lavas Basalts (2° class of Daubeny ) formed partially under water. the upper parts from not being exposed to its action is often columnar. — Dr Daubeny does not assent to this: it appears however to agree well in this case, although to me inexplicable. —
1833 Jan: In [Loreote]. the Dolerite where enters the sea is prismatic Von Buch.1
See in the posterior current p. 14 probably formed after elevation & with [cui]. — Basalt columnar. —
1832 Jan — Feb St. Jago
Lava. — This, where it is compact, is very hard, (c) of a grey
smell colour & & earthy smell: I should think consisted of a very fine mixture of Feldspar & Augite: (12. … 15) (71….. 73)
(a) The lower surface of this rock (82) is vesicular & the upper part universally generally assuming the form of concentric balls (18 … 20) & full of cavities: sometimes as described it is earthy at [word deleted] others crystalline. Frequently it is irregularly vesicular (160) with narrow beds & surface of more compact rock. — This latter assumes a curved columnar form. generally where the beds are narrower. —
Its thickness has been mentioned where most regular as 50 feet & its surface about 100 above the sea. —
The thickness appears to remain constant in the different fields of table land: but varies in other parts. In Quail Island it is only about 15 feet; round Flag Staff hill it is much thicker then 50. — (b)
Rock of the same character in every point, extends for eight miles into the interior but it is not at all clear that it has all come from the same stream: as has been mentioned from pebbles found beneath the former coast. it is clear that there existed the same sort of Lava, as now composes the general covering; & presently I shall show there has been one small stream posterior to it. —
generally] added pencil.
3 strata] added pencil.
addition. strata] added pencil.
[Notes to page 28 recto]
(a) This body of water must have been a rush, & not continuous stream, as the spaces between the large stones are not filled up. —
(b) The compact lava is intimately united with its walls: I presume by its heat:
NB The height of F. Staff Hill is certainly owing to many streams of Lava.
NB. The term of vesicular I have used rather too indiscriminately; — in this case I mean a rock full of irregular cavities as if made up of angular bits joined: when I have called the upper surface of lava vesicular the cavities are in shape of air-bubbles. —
NB The height of F. Staff Hill is certainly owing to many streams of Lava.] added pencil.
1832 Jan — Feby St. Jago
These streams, where they have passed above the lower crystalline rock, have caught up & entangled pieces. but when
they have over the former beach which abounds in Lime. [word deleted] there is found considerable quantity of Arragonite. (160 …. 163) crystallizing either in radiating balls or mammillary masses. — The posterior stream alluded to is seen in the coast E of Praja. — An immense body of water has fairly cut through the covering of Lava & former beach (a). — leaving at the bottom a thick bed of large boulders. such as are the composed of the concentric balls which are so universal in the surrounding table land & a light cavious rock of a red colour such as exists in a neighbouring hill, the Flag Staff hill. —
Lying on this bed is a thicker one of a compact & columnar lava (159), such as occurs in many parts of the Island: This cross stream is rendered more striking by the (b) neighbouring lava on each side not being compact but highly vesicular. [marginal at this section: ?] The stream has [here] come from the NE in the direction of Flag Staff hill, which is distant about a mile & a half: I
can think it almost certain that it is from this source that it has flowed. — The remarkable occurrence of Tufa with a white calcareous
[here]] added pencil.
[Notes to page 29 recto]
(a) melts easily into a black glass
(b) The rock is changed into a red colour as likewise is the upper part of the bed of boulders
(c) this tufa is composed of angular fragments
1832 Jan — Feby St. Jago
base amongst the Lava. renders this still more probable, since it occurs mixed in same manner in the Flag Staff hill: — Further to the East the table land is capped with the same sort of compact lava. — the line of separation very well defined by the effect produced on the inferior bed. — (b)
This likewise comes
in from direction of Flag Staff hill & appear only to differ from this last in not having a channel cut. —
In several places in the Island conical or rounded hills rise through the general covering of Lava. — Red hill W of Praja &
600 600 feet high may be considered a type of this class: The field of lava gradually rises in height & become irregular as it approaches the base. — It likewise changes its character becoming extremely full of cavities of little specific gravity & of a red colour. — (110 …. 115) (a) On the higher parts it is mixed with beds of Tufa, formed of same substance in a white friable calcareous base. — (116 … 120) This latter substance sometimes forming beds by itself. — (c)
This hill has a rounded form & in one side there is a large oral cavity: which I cannot help thinking may have some connexion with an antient crater.
one would certainly think, this Lime have been ejected with fragments. — do not believe hill submarine
one would certainly...submarine] added pencil.
[Notes to page 30 recto]
(a) 7 or 8 miles
(b) As far as I could observe, not having ascended to the top of the Northern one. —
(c) Here the former coast is higher perhaps 70 feet & rests on a conglomerate. chiefly the result of delution & decomposition of Augite rocks. —
(e) The hill is on a small point & promontory
& as the coast is on three side I think it clear that this hill burst up through it in a melted form. — We may hence perhaps imagine that most the rocks were formed beneath the sea. —
(d) The conglomerate occurs not far off on a table Land
1832 Jan — Feb St. Jago
Everything leads me to suppose that this has been a centre of eruption. I do not think it the only one or the greatest. — The mountains around St. Domingo show appearances of causes much more adequate to the effect produced. —
The plain can almost be traced beneath ejections of Flagstaff Volcano
Two other hills. one small one on road to Ribera Grande & another lofty one North of Praja. (a) conical & tapering in their forms
appear to belong to the same class as Red Hill: They differ in the absence of Tufa. — (b)
Dikes in Flagstaff Hill
Flag Flag Staff Hill is worthy of a particular description, it unites the characters of the Augitic hills (Page 11) and the scorious ones such as Red Hill. — This Flag Staff hill is 450 feet high. & overhangs the sea. the lower half is cut into a perpendicular face. — This part is like the cliffs round Praja only more crystalline & compact.
all belongs to modest series
The former beach run
one on each side of it in its usual horizontal course. (c) but as it approaches this part it is on each side gradually lowered. (e) till hidden by present sea. — that is, it has not been upraised equally with the other fracts. — The top part consists of alternations of black crystalline rock. Tufa with red & calcareous base conglomerate & a lava on scoria dipping to NE. — (121 … 125) (d)
The plain can almost be traced beneath ejections of Flagstaff Volcano] added pencil.
Dikes in Flagstaff Hill] added pencil.
all belongs to modest series] added pencil.
[Notes to page 31 recto]
(a) To the north of the hill is narrow precipitous gulley. its sides must be nearly 300 feet & width not more then half. — It terminate at one end in the sea. the other abruptly. — it has certainly has not been formed by water: but ought rather to be considered as a crack formed during the upraisal of the table land. — By the agency of water it would soon form a valley similar to the usual ones in the island. — the lower rocks would easily wash away & the covering of lava then would break down in masses, which afterwards would be washed in the shape of pebbles into the sea. — I should conceive the gulley is similar to the Barrancos in Madeira
By supposing this hill to have
originally existing before table land former beach was formed everything will be explained
its weight preventing greater upheaval
By supposing this...greater upheaval] added pencil.
1832 Jan — Feb St. Jago
& in one place apparently dipping to the west. — I should think their appearances were produced at the same period that the posterior current of Lava flowed from this hill. — (Page 14) It has been stated that the hill has been less upraised than other parts of the coast & yet the superincumbent lava is much thicker & of a different character. — I must likewise suppose this to have been a centre of eruption. — (a)
Before describing the valleys I will mention the very little I know of the more lofty central chain of mountains. — Near Fuentes, a village about 11 miles north of Praja there a large precipitous hill consisting of numerous beds of
vesicular cellular lava, scoriae & tufa: there on Eastern side dips very regularly to NE. St. Domingo a principal village in St. Jago lies to the West of Fuentes & is situated in an amphitheatre of black & nearly precipitous rocks about 500 feet high. — The surrounding country bespeaks the utmost violence; pieces of rocks torn apart from their beds stand like castles walls erect. — others as wild & jagged in their outlines as mica slate give to the horizon a grand & picturesque appearance. — The effect is rendered more striking by the beautiful contrast of the valley, where Orange, Banana. Cocoa-Nuts are flourishing together in their luxuriance. —
31 verso [blank]
1832. Jan — Feby St. Jago
The rock consists of vitreous feldspar & reddish crystals. (154 & 155) the fracture is angular & flaky:
it sometimes assumes a columnar form but generally is arranged in a numerous horizontal beds. —
fine series ??
I conceive it to be clear, from the pieces left standing & from the corresponding appearance on each side of the valley that the country was originally covered with an uniform bed of this rock — & that after being shattered by some great force, their valleys were formed by the agency of large bodies of water: To this latter force the valleys nearer the coast give abundant evidence. —
Following up a brook for a mile north of St. Domingo the lowest section gave a rock of [space left blank] with mica & dividing into balls as big as a pistol bullet. — (156. 157) On this was a thin bed of pebbles. then a rather thicker one of columnar basalt, (158) on which rested the great bed (several hundred feet) of same formation as around St. Domingo.
All the geological phenomenon hitherto mentioned including the last one. the upraising of the former coast date from an earlier period. than the formation of the valleys: there are indeed traced of diluvium. on the table land, which must from the nature of the pebbles have been deposited. there before they were raised at the present level. —
On nearly all the plains thonilite pebbles
modern] added pencil.
fine series ??] added pencil.
On nearly all the plains thonilite pebbles] added pencil.
[Notes to page 33 recto]
(a) There are many isolated patches of table land. —
(b) Upon considering the fact that the upper parts of the great bed of diluvium are cemented into a conglomerate: I presume it must have remained some time in water not violently agitated: & contrary to my first opinion that this valley was to be attributed to bodies of fresh water …I should think it most probable that it was formed by the sea before the coast was upheaved. — Probably the posterior valleys, which cut these were formed by
the ordinary torrents of water from the interior & which would account for the valleys dividing & terminating in the usual. which would not be the case if both sorts of valleys were formed by the sea. —
(d) this posterior valley cuts trough the former diluvium.
Read this at page (21) instead of the note (?) there
(d) this posterior valley cuts trough the former diluvium.] added pencil.
Read this at page (21) instead of the note (?) there] ink, vertically in left margin.
1832 Jan — Feby St. Jago
The valleys are a remarkable feature of this Island. being both numerous & traversing the Table land in every direction. —
It is to this cause (a) that the truncate conical are owing — on such a one the town of Praja is built & Quail island may be considered as another. — The valleys are generally broard with a flat bottom of diluvium & with abrupt sides. — this latter fact is explained on the same principles as the cliffs viz. the upper Lava resisting decomposition & violence much longer than the lower Augitic rocks: — I am decidedly of an opinion that these valleys were formed by great bodies of water & not by gradual effects. — the mere flatness of their bottoms gives it a strong presumption: During the upheaving of so many miles of coast it is probable that there would be long rents & of course especially in those places where any irregularity occurs in the elevation. — It is in such situations that the valleys occur: the one that forms Quail Island is a good instance of this. — These valleys are of two
sorts ages: one broard one runs NE. & has at his bottom a bed of diluvium (b) at least 20 feet thick. the upper part of which is cemented into a conglomerate. — (d) other valleys cross this, the road to Trinidad is one of them running NW. —
[Notes to page 34 recto]
Sir G. Staunton mention another 34 feet
vide Macartney Embassy
The 2d or modern set of valleys are broard & flat, not like an ordinary one ! —
probably united effects of fresh water & lava to retiring sea.
(a) In this occurs large masses of the crystalline vitreous Feldpar & in a situation that it is almost impossible to account for its presence excepting by changing the level of the ground. —
Yet this change affects all equally
Sir G. Staunton mention another 34 feet] added pencil.
The 2d or modern set of valleys are broard & flat, not like an ordinary one ! —] added pencil.
probably united effects of fresh water & lava to retiring sea.] added pencil.
Yet this change affects all equally] added pencil.
1832 Jan — Feb. — St. Jago
In this grows the celebrated Baobab or Adansonia; this tree is only 45 feet high, measured two feet from the ground round the solid trunk. 35. — Some of the same species in Africa were supposed by Adanson1 to reach the enormous age of 6000 years. — The very appearance of the tree strikes the beholder that it has lived during a large fraction of the time that this world has existed. — Of course the valley must be still older & it is this one that has
tot finally left the neighboured of Praja in the state we now find it. — How long a time intervened between this period & the deposition of former beach, it is impossible to say. — during it three great phenomena occurred. the flowing of the Lava. the upheaving of the coast. & the great beds of diluvium coll collected by in the older valley. — To what a remote age does this in all probability call us back & yet we found the shells themselves & their habits the same as exist in the present sea. — Before leaving this subject (a) I must mention part of a bed of diluvium at a higher level than the pap -form hills. & indeed resting on them. — I can only account for this by supposing it was then deposited previous to the general upheaving of the Island. —
1 Michel Adanson (1727–1806). French naturalist and traveller.
[Notes to page 35 recto]
(a) It certainly sounds the most probable explication. — that when the former coast was upheaved, the valleys were formed by the draining off of the sea. — but in this case we must suppose the beach abounding with shells to have been formed at a great depth. — sufficient to cover the superimposed bed of lava, & to then to allow
of a body of water enough to make a torrent when the island was lifted up. — Shells at such depth are I believe not common. —
Again we have evidence, that there has been one stream sufficient to cut through the Lava & former beach. Leaving a bed for the
posterior lava posterior to the general covering. — this then is a vera causa. —
Vide note to P. 19
The place where Lava is cut trough might have been on Barranca such as described near F Staff Hill. —
Vide note to P. 19...near F Staff Hill. —] added pencil.
1832. Jan — Feb. St Jago
(a) Whilst viewing these valleys. one speculates how to account for the agency of so much water, in a country so remarkable for its dryness as St. Jago is. — I thought it might have been the sea. — but the valley terminate & divide in so usual a manner, that I was obliged to give up this idea. & go back to the torrents of rain that usually are said to accompany volcanic action. —
The best geological fact I have to notice, is the forming at the present day of a remarkably hard conglomerate. When breaking it, I was forcibly reminded of the very tough conglomerates of the Old Red sandstone formations. It occurs very commonly on different parts of the coast. the pebbles are the Augitic rocks & amigdaloid & of the superior Lava) 35 & 36, Phonolite) (75 & 76 & 195). The matrix slightly efferveces with Acids & easily melts under the blowpipe into a black glass. — From what. I could observe it is the clayey matter, resulting from the decomposition of the augitic rock mixed with Carb. of Lime perhaps from the former beach, where it is so abundant. — Would not this form a natural [illeg]stone ? — I have the clearest proof that this conglomerate is forming at the present time. since it not only contains shells
Phonolite)] added pencil.
[Notes to page 36 recto]
(a) If as I suppose the shells in the former beach are the same as now exist. — the superimposed lava comes under the class of formations of the present day. — Dr Daubeny states that those Lavas, which like this, are in their composition Basaltic & form fields rather than circumscribed streams
were originated at the time when Tertiary formations were depositing. — does not this instance war against the rule. —
1832 Jan — Feb St Jago
but likewise pieces of brick & bolts of Iron;
so the latter so firmly embedded that I could by no means detach them. —
(a) Before concluding this sketch of the island I must add I think it would well repay the trouble of a more
<c. 4 lines excised>
The most interesting facts for future observation. are the extent of former coast to the East. Flag Staff hill. — The central chain & the relation of pap-form hill & Trachytic to the other rocks. —
Effect of Lava on shells in former coast. —
Effect of Lava on shells in former coast. —] added pencil.
Are the central hills traversed by dykes? yes
Rereading this paper: (Notes 1) old coast must have been been covered with Lava. before elevation. otherwise the loose ingredient of former beach would have been washed away.
(2) I do not know whether Quail Island was formerly an island or whether former beach was removed before its covering. — the inclined position of fragment of coast (K) shows subsid.
<c. 4 lines excised>
(4) The hills of scoriae. as Red Hill must compare to the cones of Etna, which never have sent of Lava & around which the field of lava from obstruction is higher
??? F Staff has sent forth
(5) The prevalence of conglomerates in ancient lava being thought characteristics of their age. it is curious to see a large & perfect one forming at the present day. —
(6) The friable lime is Tufa resemble lime which has been slacked & absorbed Carbonic acid water. —
I do not see my proof of the pre existence of the lava to the general covering.
x 2 If the beds had been washed away, the inferior ones would have covered the edges of crystalline rock, instead of regular succession: clearly showing (K) was a point (island or peninsula) round which its beds were successively deposited:
& from weight less elevated, like F. Staff Hill
Quail Isld one thinning out & capping it
Are the central hills traversed by dykes? yes] added pencil.
??? F Staff has sent forth] added pencil.
I do not see my proof of the pre existence of the lava to the general covering. ] added pencil.
& from weight less elevated, like F. Staff Hill] added pencil.
Quail Isld one thinning out & capping it] added pencil.
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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 22 March, 2013