To J. S. Henslow1 18 April 1835
My dear Henslow.—
I have just returned from Mendoza, having crossed the Cordilleras by two passes. This trip has added much to my knowledge of the geology of the country.2 Some of the facts, of the truth of which I in my own mind feel fully convinced, will appear to you quite absurd & incredible.— I will give a very short sketch of the structure of these huge mountains. In the Portillo pass (the more Southern one) travellers have described the Cordilleras to consist of a double chain of nearly equal altitude, separated by a considerable interval.— This is the case: & the same structure extends to the Northward to Uspallata; the little elevation of the Eastern line (here not more than 6000–7000 ft), has caused it almost to be overlooked. To begin with the Western & principal chain; we have where the sections are best seen, an enormous mass of a Porphyritic conglomerate resting on Granite. This latter rock, seems to form the nucleus of the whole mass & is seen in the deep lateral valleys, injected amongst, upheaving, overturning in the most extraordinary manner the overlying strata. On the bare sides of the mountains, the complicated dykes & wedges of variously coloured rocks are seen traversing in every possible form & shape the same formations, which by their intersections prove a succession of violences. The stratification in all the mountains is beautifully distinct & from a variety in the color can be seen at great distances. I cannot imagine any part of the world presenting a more extraordinary scene of the breaking up of the crust of the globe than the very central peaks of the Andes. The upheaval has taken place by by a great number of (nearly) N & S lines; which in most cases has formed as many anticlinal & synclinal ravines: The strata in the highest pinnacles are almost universally inclined at an angle from 70o–80o.—
I cannot tell you how I enjoyed some of these views.— it is worth coming from England once to feel such intense delight. At an elevation from 10–12000 ft. there is a transparency in the air & a confusion of distances & a sort of stillness which gives the sensation of being in another world, & when to this is joined, the picture so plainly drawn of the great epochs of violence, it causes in the mind a most strange assemblage of ideas.
The formation I call Porph-Conglomerates, is the most important & most developed one in Chili; from a great number of sections, I find it a true coarse Conglomerate or Breccia, which by every step in a slow gradation passes into a fine Clay-stone Porphyry; the pebbles & cement becoming Porphyritic, till at last all is blended in one compact rock. The Porphyries are excessively abundant in this chain. I feel sure at least of them have been thus produced from sedimentary beds in situ.— There are Porphyries which have been injected from below amongst Strata & others ejected which have flowed in streams: it is remarkable I could show specimens of this rock, produced in these three methods, which cannot be distinguished. It is a great mistake considering the Cordilleras (here) as composed of rocks which have flowed in streams. in this range I no where saw a fragment, which I believe to have thus originated, although the road passes at no great distance from the active Volcanoes.—
The Porphyries, Conglomerates, Sandstones & Quartzose Sandstones, Lime stones alternate & pass into each other many times (overlying, where not broken through by the Granite, Clay-Slate) In the upper parts the Sandstone begins to alternate with Gypsum, till at last we have this substance in a stupendous thickness. I really think the formation, is in some places (it varys much) nearly 2000 ft thick. it occurs often with a green (Epidote?) siliceous Sandstone & snow white marble: it resembles that found in the Alps in containing large concretions of a crystalline marble of a blackish grey color.— The upper beds, which form some of the higher pinnacles consist of layers of snow white gypsum & red, compact sandstone, from the thickness of paper to a few feet, alternating in an endless round.— The rock has a most curiously painted appearance.—
At the pass of the Puquenas in this formation, where however, a black rock, like Clay-Slate, without many laminæ occurring with a pale Limestone has replaced the red Sandstone, I found abundant impressions of shells.— The elevation must be between 12–13000 ft.— A Shell which I believe is a Gryphæa is the most abundant,—an Ostræa, Turritella, Ammonites, small Bivalves, Terebratula (?).— Perhaps some good Conchologist will be able to give a guess, to what grand division of the formations of Europe, these organic remains bear most resemblance.—3 They are exceedingly imperfect & few. the Gryphites are most perfect.— It was late in the Season, & the situation particularly dangerous for Snow storms. I did not dare to delay, otherwise a grand harvest might have been reaped.—
So much for the Western line; in the Portillo pass, proceeding Eastward we meet an immense mass of a Conglomerate dipping to the West 45o, which rests on Micaceous Sandstones &c &c, upheaved, converted into quartz rock, penetrated by dykes, from the very grand mass of Proto gine4 (large crystals of quartz, red Feldspar & occasional little Chlorite). Now this Conglomerate, which reposes on & dips from the Protogine
45o, [GREATER THAN]
consists of the peculiar rocks of the first described chain, pebbles of the black rock with shells, green sandstone &c &c: It is hence manifest, that the upheaval (& deposition at least of part) of the Grand Eastern chain is entirely posterior to the Western. To the North in the Uspallata pass we have also a fact of the same class.— Bear this in mind, it will help to make you believe what follows.—
I have said the Uspallata range is geologically, although only 6000–7000 ft a continuation of the grand Eastern chain.— It has its nucleus of granite, consist. of grand beds of various crystalline rocks, which I can feel no doubt are subaqueous lavas alternating with Sandstone, Conglomerates & white Aluminous beds (like decomposed feldspar) with many other curious varieties of sedimentary deposits. These Lavas & Sandstones alternate very many times & are quite conformable, one to the other. During two days of careful examination I said to myself at least 50 times, how exactly like, only rather harder, these beds are to those of the upper Tertiary strata of Patagonia, Chiloe, Concepcion, without the possible identity ever having occurred to me.— At last there was no resisting the conclusion.— I could not expect shells for they never occur in this formation; but Lignite or Carbonaceous shale ought to be found. I had previously been exceedingly puzzled by meeting in the Sandstone thin layers (few inches to feet thick) of a brecciated Pitchstone I strongly suspect, the alteration, from the underlying Granite, has altered such beds into this Pitchstone. The silicified wood, (particularly characteristic) was yet absent. the conviction that I was on the Tertiary Strata was so strong, by this time in my mind, that on the third day, in the midst of Lavas, & heaps of Granite I began my apparently forlorn hunt.—
How do you think I succeeded? In an escarpment of compact greenish Sand-stone I found a small wood of petrified trees in a vertical position, or rather the strata were inclined about 20–30 to one point & the trees 70o to the opposite one.— That is they were before the tilt truly vertical.— The Sandstone consists of many layers & is marked by the concentric lines of the bark (I have specimens)5 11 are perfectly silicified, & resemble the dicotyledonous wood which I have found at Chiloe & Concepciòn: the others 30–40 I only know to be trees from the analogy of form & position; they consist of snow white columns Like Lots wife of coarsely crystall. Carb. of Lime. The longest shaft is 7 feet. They are all close together within 100 yd. & about same level; no where else could I find any.— It cannot be doubted that the layers of fine Sandstone have quietly been deposited between a clump of trees, which were fixed by their roots.— The Sandstone rests on Lavas is covered by great bed, apparently about 1000 ft thick of black Augitic Lava, & over this, there are at least 5 grand alternations of such rocks & aqueous sedimentary deposits; amounting in thickness to several thousand feet.— I am quite afraid of the only conclusion which I can draw from this fact, namely that there must have been a depression in the surface of the land to that amount.— But neglecting this consideration it was a most satisfactory support of my presumption of the Tertiary (I mean by Tertiary, that the shells of the period were closely allied or some identical to those which now live as in lower beds of Patagonia) age of this Eastern Chain.
A great part of the proof must remain upon my ipse dixit, of a mineralogical resemblance, with those beds whose age is known, & the character of which resemblance, is to be subject to infinite variation, passing from one variety to others by a concretionary structure. I hardly expect you to believe me, when it is a consequence of this view that Granite which forms peaks of a height probably of 14000 ft has been fluid in the Tertiary period.—that strata of that period are altered by its heat & are traversed by dykes from the mass: That these Strata have also probably undergone an immense depression, that they are now inclined at high angles & form regular or complicated anticlinal lines.— To complete the climax & seal your disbelief these same sedimentary Strata & Lavas are traversed by very numerous true metallic veins of Iron, Copper Arsenic, Silver & Gold, & that these can be traced to the underlying Granite.— A Gold mine has been worked close to the clump of silicified trees.—
If when you see my specimens, sections & account, you should think that there is pretty strong presumptive evidence of the above facts: It appears very important: for the structure, & size of this chain will bear comparison with any in the world. And that this all should have been produced in so very recent a period is indeed wonderful. In my own mind I am quite convinced of the reality of this. I can any how most conscientiously say, that no previously formed conjecture warped my judgement. As I have described, so did I actually observe the facts.— But I will have some mercy & end this most lengthy account of my geological trip.—
On some of the large patches of perpetual snow I found the famous Red Snow of the Arctic countries.— I send with this letter my observations & a piece of Paper on which I tried to dry some specimens. If the fact is new, & you think it worth while, either yourself examine them or send them to whoever has described the specimens from the North, & publish a notice in any of the periodicals.— I also send a small bottle with 2 Lizards: one of them is Viviparous, as you will see by the accompanying notice.— A M. Gay, a French Naturalist has already published in one of the Newspapers of this country a similar statement, & probably has forwarded to Paris some account:6 as the fact appears singular, would it not be worth while to hand over the Specimens to some good Lizardologist & Comparative Anatomist to publish an account of their internal structure.— Do what you think fit.7
This letter will go with a cargo of Specimens from Coquimbo.— I shall write to let you know when they are sent off.— In the Box, there are two Bags of Seeds, one ticket, Valleys of Cordilleras 5000–10000 ft high; the soil & climate exceedingly dry; soil Very light & stony, extremes in temperature: the other chiefly from the dry sandy Traversia of Mendoza 3000 ft more or less.— If some of the bushes should grow but not be healthy try a slight sprinkling of Salt & Saltpetre.— The plain is saliferous.— All the flowers in the Cordilleras appear to be Autumnal flowerers,—they were all in blow & seed—many of them very pretty.— I gathered them as I rode along on the hills sides: if they will but choose to come up I have no doubt many would be great rarities.— In the Mendoza Bag, there are the seeds or berrys of what appears to be a small Potatoe plant with a whitish flower. They grow many leagues from where any habitation could ever have existed, owing to absence of water.— Amongst the Chonos dryed plants, you will see a fine specimen of the wild Potatoe, growing under a most opposite climate & unquestionably a true wild Potatoe.— It must be a distinct species from that of the lower Cordilleras one.— Perhaps, as with the Banana, distinct species are now not to be distinguished in their varieties, produced by cultivation.— The Beagle is not at Valparaiso. So I cannot copy out the few remarks about the Chonos Potatoe.—8 With the Specimens, there is a bundle of old Papers & Note Books. Will you take care of them, in case I should loose my notes, these might be useful.— I do not send home any insects, because they must be troublesome to you & now so little more of the Voyage remains unfinished I can well take charge of them.—
In two or three days I set out for Coquimbo by Land, the Beagle calls for me in the beginning of June: So that I have 6 weeks more to enjoy geologizing over these curious mountains of Chili.— There is at present a bloody revolution in Peru: the Commodore has gone there & in the hurry has carried our letters with him; perhaps amongst them there will be one from you.— I wish I had the old Commodore here I would shake some consideration for others into his old body.— From Coquimbo you will again hear from me.— Till then Farewell. My dear Henslow— Yours very truly, C. Darwin
Our plans are altered. I have a ten weeks holiday & expect to reach as far as Copiapò & examine all that preeminently curious country abounding with mines:—
I shall not write to you till we reach [left blank] excepting half a dozen lines, just to inform you when my specimens leave this Port.— I am glad to say, that I believe this will be the last Cargo, with which you will be troubled.—
Has just returned from crossing the Cordilleras. Geological observations of formations representing great epochs of violence which broke up the earth’s crust. Shells at over 12000 feet. Silicified trees in sandstone formations at great heights. Red snow and viviparous lizards. Botanical specimens.