Darwin’s geological notes, 1835 – 1842: Recapitulation and concluding remarks.
The notes are mostly in Syms Covington’s hand. For image view in the Cambridge University Digital Library, click CUL-DAR 41.23-39. For transcription of R N (Red Notebook), see F1583. For scans of the Red Notebook, mostly here: https://cudl.lib.cam.ac.uk/view/MS-DAR-00042/3
Transcribed by Christine Chua (05.2024). Free to use, credit appreciated. With thanks to Greg Priest for the corrections.
(1) Recapitulation and concluding remarks
Before finally leaving the shores of South America, I will recapitulate those conclusions and facts which appear to me to be most worthy of attention.
With respect to the Cordillera of the Andes, that great axis of the whole continent, my examination of its structure has been chiefly confined to a space of 400 miles of Latitude in central and northern Chili. The formations were there arranged under five principal divisions. Before I offer a few observations on each class, I must distinctly state, that these divisions are not clearly defined, but they are only to be distinguished, when we imagine ourselves viewing the mountains from a distance. Even within the limited space of northern and central Chili, the relative thickness and several other characters of each formation are subject to considerable variation; yet from the little I have seen and heard of other parts, I suspect that the same divisions are applicable to a great extent of the Cordillera
Form 1st and IInd. The lowest formation is granitic; it is covered by a second formation, composed by more or less crystalline slaty rocks. These two generally form the shores of the Pacifick; it is only by travelling some way inland, that the escarpment of the superior formation is met with. For a space of 1300 miles, from the South point of the peninsula of
[1v]
(De la Beche paper on Jamaica)
(speculations on method of S. American Elevation R. N. p 11)
(Versus Sir J. Herschel ideas of first Cause R. N. p. 32)
(In Europe limited volcanos & limited earthquake, limited but great local elevation contrast with America)
(Granitic conglom: generally between oldest & most modern rocks in all Europe R. N 110)
(Good abstract of Humb. S. America Geology in Daubeny p 349)
(Huge enormous quantity of matter from Andes)
(Lyell states that East India Volcanoes characterized by abundance of Sulphuric acid vapours
(R N. p. 48 & 49 on effect of elevation [sketch])
(R. N. p 51 & p. 54 absence of secondary formation near in S. America)
(Speculate on symmetry of world R N p 123)
(conical masses of greenstone p 88)
(on Theory of Volcanoes R. N. p 57) (Simplicity of world R. N. p. 72)
(Method of elevation: dikes: not both p 58-59) (p 68 & 69) (p 75, 76)
(R N. process of elevation in form of land p. 61 – p 64)
(Shores of the Pacific compared to whole East coast N & S. America, Africa Australia, previously deep grand faults)
(RN 71) (grandeur of action in Cordillera)
(Speculation on Volcanos & metallic veins R. N. p. 78) – (p 111 & 112) → no metals in Polynesian islets.
(Quartz pillar in Cordillera look like elevation)
[Water-coloured map of South America, southern third only, DAR41.24a-24b]
Tropic of Capricorn. Lat. 22.33
Copiapo
Concepcion
Chiloe
C. Tres Montes
Sts of Magellan
C. Horn
Staten land
S. Cruz
P. Desire
Bay of St Joesph
R. Negro
Buenos Ayres
R. Plata
R. Grande
St Catherines
Falkland Isds
[Map legend]
[Blue] Tertiary Strata of all epochs
[Pink] Granite. Gneiss. Mica. Slate. Quartz R on Clay Slate
[Purple] Subaqueous. & Aerial Lavas & their Tufas & detritus: chiefly Porphyries. – Also any substance which has proceeded from Volcanic origin: such as the Gypseous Strata
(2) Recapitulation and concluding remarks
Tres Montes to Copiapo, there is no exception to this law… The whole coast of Tiera del Fuego, an extent of 250 miles, is of similar constitution. To the north of the Straits of Magellan towards Tres Montes, specimens collected during Capt. Kings voyage, and now deposited in the Geological Society, will throw some light on the geology of that part. To return to the north of Chili; at Cobija we meet the same granitic rocks, whilst in the southern Peru (Iquique & Arica) the escarpment of Porphyritic Breccia (Iquique & Arica),fronts the sea.
[in margin] (at Payta M. Lesson says there are [?]alcose slates)
At Islay however again the usual granitic land appears. Amongst the unstratified rocks granite and Syenite are both common, and are sometimes accompanied by greenstones and granitic porphyries.
In the second formation, the laminated rock are either micaceous schist, gneiss and phyllade or a green hard felspathic (altered clay) state. This latter, commonly replaces the former kinds and is distinct from them; yet they have occasionally been seen to graduate into each other. I am ignorant what circumstances have determined their respective appearance. Unaltered clay-slate I have but rarely met with on this coast. These two crystalline formations are frequently auriferous; and especially the alluvium produced by their degradation. The axis of Cordillera consists of Syenitic Greenstone associated with some true granite, the chief character of the former
[2v]
In describing structure of Cordillera line of elevation has connected points of Eruption
(Probable as eruptions constantaneous same arch of melted matter ∴ same ridge of mountains)
(viz, give instances Etna Stromboli & Vesuvius if elevation then these mountains probably would be connected.) (order of ejected lavas A31)
Earthquake in Cordova R N. p 142 & 149 Temple’s Peru?
in Tucuman R N. p 157
(Coast of Brazil being elevated is against theory yet facts seem probable
(Conybeare recommend me to read Silliman Journal 1835. N. American Geology. —
(In Cordillera. besides detritus carried on each side elevation, showing great afflux of melted matter to that line)
(Alpine country extends far into N Pampas, RN 155 like Chili to South)
(Remark on abundance of Coniferous woods in S. America constant with recent vegetation, Count specimens)
(My theory of metallic veins RN p 165)
(Mexico Trachyte called andrite A26 — (geology of Mexico RN. p 171) (p 176)
(Study in Humboldt Fragmens Asiatiques an account of great american eruptions) — (Quito Volcanoes A. p 28)
(Cayenne Syenite &c A. p. 20) — (Granite at Essequ…. A p. 30
(3) Recapitulation and concluding remarks
is the absence of quartz; its age must be posterior to the strata it has upheaved, and as I believe likewise to the ordinary and more perfectly characterized granite of the coast.
This conclusion agrees with a similar one of Humboldt (?). Looking to the Eastern side of this part of the Cordillera; we have at their base near Mendoza both large clay-slate, and a mica slate formation, perhaps also an ancient granite. At the River of Santa Cruz judging from fragments there are altered felspathic and some chloritic slates. In Tierra del Fuego to the east of the prolongation of the grand chain, there is an extensive district of clay-slate.
Extending our view towards the Atlantic, we see the auriferous granites and granitic slates, which constitute the enormous platform of Brazil and La Plata; In the latter country a part of the ranges of Cordova terminate at San Luis de la Punta, only sixty leagues to the east of the Andes.
In northern Patagonia and in the Falkland Islands the formations are of quartz and clay slate, on the coast of central Patagonia patches of similar rocks and micaceous slates occur. I look at these (a) formations, now found on opposite shores, as having once, but perhaps not in their present condition, constituted the floor of a great ocean, before the volcanic powers had commenced rearing the Cordillera.
[3v]
(a) Note, I should believe the Falkland Islands Quartz Rock to be of same age with clay-slate of Tierra del Fuego; then, if my views are correct the fossils found there, ought to indicate a higher antiquity than those of the Porphyritic Breccia formation of Chili.
on passage of Porphyry from granite to volcanic rocks p. 236 Daubusson)
When discussing subterranean currents, similarity of hypogene rocks over world may be mentioned.
————
Humboldt Superposition studied
————
Granite in alp & Pyrenees lies without central chain
————
In Henslow’s papers dikes generally transverse to cleavage (NE & SW)
————
Mem – Gneiss, Quartz at Patagonia
(Granite at Acapulco)
(On Abiotic lavas of Andes. von Buch Lyell vol II p 175)
(On grandeur of formation on coast of S. America. all granitic
no rivers Peru? all sediment nearly same: action of beach cause of sediment)
(Lyell vol IV p 224 shows that some inclination is given to strata after change into dry land)
Abich in his geological views of ∠ Vesuvius & Etna, states that small crater opened on the side of fissure 20 ft high formed by an elevation; on the side which had not been raised, Plate 1
(4A) Recapitulation &c.
This view explains the linear grouping and confinement of the formation. – In a deep ocean such lava and tufa would be piled on each side of the lines of eruption.The period of time, during which this mound was accumulating must have been very great. At St. Jago I believed the formation to be one mile thick; this sounds immense, one figures to oneself a ridge beneath the water with steep sides. It must however be remembered that this thickness is not probably more than one sixtieth part of its breadth. – That the porphyry lavas flowed from out of a granitic floor, the presence of dikes and isolated hills, now surrounded by such formations, tends to show – The lavas and their tufas probably often covered the Clay-slates, whilst the latter were forming and before their consolidation.
Hence we have a gradual passage from one to the other class. I have no means of conjecturing at what period the great metamorphic action commenced, to which the granitic rocks owe their origin. In the inferior parts, the change might have been effected long before the first eruption of porphyry lava happened; or it might be connected with the internal movements,
(4) Recapitulation and concluding remarks
Form III. We now come to the (III) division, the escarpment of which is generally found some little way inshore, I have repeatedly referred to the appearance and origin of the Porphyritic Breccia; and of the general accuracy of those accounts I feel no doubt. The main kind of rock, is a dull purple claystone porphyry – which I have shown has generally flowed as a subaqueous lava, under high pressure. The tufa, resulting from the ejected angular and partially rounded fragments has been subsequently recemented by the action of heat. In these mountains the author of the Huttonian theory might have revelled in facts giving full support to his views. The thickness of the formation varies; near St. Jago perhaps it amounts to 6000 feet, whilst in northern Chili it does not, probably exceed the half of this. At the former place rocks bearing evidence sometimes of a mechanical and sometimes of a crystalline structure preponderate, at the latter alternations of lava and conglomerates are distinctly manifest.
The orifices, from which the porphyries flowed, were arranged over a space, which in one direction, was excessively long and in breadth from fifty to a hundred miles.
I have no means of conjecturing, at what periods the great metamorphic action commenced, to which the granitic rocks owe their origin.
In the inferior parts, the change might have been effected long before the first eruption of porphyry lava happened; or it might be connected with the internal movements
[4v]
The ancient & long continued active condition of craters in Cordillera, harmonize. (according to deduction in coral paper) with role of movement of elevation. –
Marsden Sumatra chain of mountains double or treble nearest to East West coast (argue this general structure (C. of Good Hope) India (England?) Norway)
separated by plains. —
(5) Recapitulation and concluding remarks
which caused that grand system of volcanic energy. It has been shown, that the porphyritic breccia formation has likewise been subjected, to metamorphic action, this however I certainly believe is of a posterior date to that in the granitic rocks. But as it is manifest, that those upper slates, which pass into and alternate, with the lower strata of porphyritic breccia, were not metamorphosised before the first of those eruptions, to which that whole formation is due, so we may conjecture that the hard felspathic nature of the clay slate and the blended condition of the materials of the porphyritic breccia are parts of one phenomenon. According to this view, the present state of our second formation would be owing to two distinct periods of metamorphosis.
[The following passage is crossed.]
This view explains the linear grouping and confinement of the formation. In a deep ocean such lava and tufa would be piled on each side of the lines of eruption. The period of time, during which this mound was accumulating must have been very great.
At St. Jago, I believed the formation to be one mile thick; this sounds immense, one figures to oneself beneath the water a ridge with steep sides. It must however be remembered that this thickness is not probably more than one sixtieth of its breadth. That the porphyry lavas flowed from out of a granitic floor, the pressure of dykes and isolated hills, now surrounded by such formations, tends to show. The lavas and their
(6) Recapitulation and concluding remarks
tufas probably often covered the clay-slates, whilst the latter was forming and before their consolidation. Hence we have a gradual passage from one to the other class. Of the geographical limits of the porphyritic breccia beyond a degree south of St. Jago I know nothing.
Perhaps the extreme prolongation of this formation may be found on the S.W. coast if Tierra del Fuego, where there has been a line of ancient eruption, and where purplish and pale coloured felspathic porphyries, and some curious sedimentary deposits have been found
Nor must we entirely overlook, that formation in Patagonia, (from about Latitude 44 20’ to 49°. 20’) found on the shores of the Atlantic, which consists of purple porphyries and altered tufas.
These so strikingly resemble the Porphyritic Breccia, that if the strata had been shattered and elevated, with the basal parts of the Cordillera the whole would have been identical with the basal parts of the Cordillera. At Rio Grande, in the Southern extremity of Brazil there is a porphyry formation overlying granite. I look at these facts with interest, both from their infrequency or rather intire [sic] absence on the Eastern shores of South America; and as showing how great has been the cause, and how wide its effects, which has reared up the great chain of the Andes.
In the 400 miles of Chili the porphyritic breccia can be most distinctly recognized; the characters which vary in the Northern and Southern parts, owe the greater part of their difference, not to original formation,
(7) Recapitulation and concluding remarks
but to subsequent metamorphic action.
North of Chili, at Cobija I saw a specimen, which appeared to have come from a similar formation; but there M. Dessalines D’Orbigny will have described the geology. At Iquique I recognized the same kind of series; – At Arica some specimens were of the same general character; and lastly at Lima, (1600 miles from the first point) I thought in the description of M. Rivero I could trace in the lower parts, this Chilian formation. I believe also in Humboldt’s superposition I shall be able partially to continue this investigation, although by the comparison of strata, formed as these are supposed to have been, with European formations, the account is to me (from my little knowledge on the subject) scarcely intelligible. – The upper part of the formation shows less the effect of metamorphic action, than the lower – the conglomerates being there more abundant, in proportion as the porphyritic lavas are less so. – The sedimentary beds vary much; some of them, which appear very general, are curious; they consist of particles of indurated green and red earthy substance containing crystals (unknown to me) and crystallized white specks of Carbonate of Lime. – I know not whether to attribute these to heat or original deposition. –
Humboldt has observed, that the mines in America are not generally formed in the oldest rocks. – The richest and most abundant veins of Silver, which I have seen, all lie in this part of the series, I may instance
(8) Recapitulation and concluding remarks
, Arqueros near Coquimbo.. S. Pedro. de Nolasko St. Jago. – Chanuncillo &c Copiapo. – Huantajaya and S. Rosa Iquique. – And I believe Pasco.
Peru. –
Form IV. It is likewise above these upper series, and generally within the more central Cordillera, where our (IV.) division or the gypseous formation may be expected to commence. – The strata which alternate with and contain such extraordinary quantities of gypsum are subject to an excess of variation, hardly known in any Tertiary basin. – Red sandstone, black calcareous compact clay slate, – white siliceous sandstone and a soft aluminous kind of honestone all replace each other at short distances. The cause of this will be apparent, if we picture to ourselves the circumstances, under which they were formed. – Volcanic eruptions which had previously taken place on so grand a scale, had become comparatively rare. – The space, now represented by the Porphyritic – Breccia must have been studded with nearly extinct craters, thus giving rise to an uneven bottom divided into basins. – In these mud and detritus from the wear and tear of the volcanic rocks would accumulate; and from the subaqueous Solfataras (if they may be so called) quantities of sulphur and lime must have been slowly ejected. In the 400 miles of the Cordillera, which I visited, its presence is general; I do not however feel certain, that all the so-called gypseous formations contain gypsum; although from hearing of its presence in nearly every valley
(9) Recapitulation and concluding remarks
I can scarcely doubt its continuity. At Iquique and Lima I heard of large quantities of it in the Cordillera. – In both places however and in Peru, generally a great formation of Salt appears to belong to this epoch. – It is in the lower part of this division and the higher parts of the latter, where all. Although but (yet few in number) of the fossil shells have been found.
Although distinct species are frequently found in, groups; yet I believe all belong to one period. At Copiapo almost every kind occurred together in the same mass of strata. –
In only one spot did I see any fossil coral but for this scarcity the great depth, at which these deposits are believed to have been accumulated, may well account. – It may be remarked, that throughout great thicknesses of strata, the same species of organic remains are found, hence we may conclude that such masses have been deposited within a time short in proportion to their volume. –
I imagine the fossils resemble in type those of the Cretaceous or Oolitic groups?” Do not the ammonites show that they are not Tertiary? Humboldt considers the fossiliferous strata in Cordillera of Lima to be Zechstein. – Does the ground this opinion on the resemblance of the red sandstones and porphyries?” I suppose in a mineralogical point of view, divisions (III and IV.) ought both to be included in the Pecilitic group.
We now come to the super-gypseous or the (v) and last division, which forms the superior mass of the Cordillera – The lower line of
(10) Recapitulation and concluding remarks
separation is more difficult to decide upon and more artificial than in the other classes. –
I do not certainly know of its presence, excepting in Chili. – The formation is however there, frequently of great thickness – is like the last subject to extreme variation – is sometimes a very coarsely mechanical rock – and is chiefly characterized by containing extraordinary quantities of silicified dicotyledonous wood. – The land, on which these trees flourished (a state of things now contrasted to the present!) was perhaps formed, by some Volcano, which yet continuing active had raised its head above the water. If not so, some small portions of the inferior strata must have been elevated. The conglomerates, between the Portillo and Puquenes ranges, and probably the strata of the Uspallata mountains, both of which, I believe, belong to this formation, differ remarkably from the rest, in their relative position. – They have been deposited at the flanks of the central chain, which had been as above supposed, elevated, into dry land.
The Uspallata range is moreover singular for from the size and great numbers of the streams of subaqueous lava.
With respect to the continuity, over large spaces of a similar mineralogical nature in these formations; such as in the various porphyries. – The gypsum – and the syenitic greenstone, it must here depend on a long chain of Volcanoes ejecting, the same substances at the same periods. I am aware, this is contrary to analogy drawn from some of the smaller groups of sub-aerial Volcanos.
(11) Recapitulation and concluding remarks
But on the other hand, the Andes which vomit forth on so many points Trachyte might be adduced. In Europe, the identity of nature in remote parts of some of the secondary strata has been attributed to one ocean depositing one kind of sediment. But does not this, in some instances, as in the Chalk formation depend ultimately on the same reason, as in the Cordillera. If ever it should be shown that there is any uniform resemblance in the formations of the same epoch in distant quarters of the globe, it will perhaps result from a similarity of volcanic ejections. – If we attempt to conjecture concerning the cause of this grand subterranean propulsion, first of one substance, and then of another and quite different one; we are lost in utter ignorance. – We see in it, a phenomenon depending on the constitution and laws of the central nucleus of the globe. Our surprise at the existence of any such connection ought however to be diminished, when it is recollected that volcanic mountains. separated by the enormous interval of fifty three degrees of latitude, have manifested an allied action.
It is hopeless even to attempt to understand this mere simple case of a tendency to eruption, reaching over so great a part of a Semi-meridian.
Having discussed the constitution of the formations, we now enter on the position of their strata. In the whole four classes of stratified rocks, I have no where seen any one stratum superposed unconformably to another.
(12) Recapitulation. &c
I except the Portillo and Uspallata beds, belonging to the last division, which lie on the flanks of the older series. This regular superposition, extending through so enormous a thickness of strata, from the superior coarse conglomerates or sandstones, to the altered clay-slates, and from these again into the micaceous schists, is one of the most remarkable features in the geology of these mountains. In the lowest beds of the porphyritic breccia, we do not find fragments of those underlying rocks, commonly called primitive. This fact, I should apprehend, is one of extreme infrequency; it is by itself a strong confirmation to our belief in the conformable stratification of the formations. We may therefore safely conclude that no general angular movements had been effected much before the deposition of the upper formation. – But then commenced an important period in the history of the Cordillera. Then it was that the strata were broken and tossed like ice, on the Arctic ocean. In crossing the great central ranges, every where signs the most enormous violence are apparent, The inclined or vertical strata, interlaced with stony dikes have their very chemical composition altered by the subterranean heat. If we look to the immediate cause of these effects, we find it in the mountains of Syenitic greenstone which compose the axis of the lines of disturbance.
(13) Recapitulations &c.
This peculiar rock, associated with granite, is general in all parts which I visited; From the description of Humboldt, I should suppose it equally so in the Mexican and Equatorial Andes. This is the rock, which I have said intirely [sic] differs from the coast granite; its age or rather the period, when in a partially fluid state, it was propelled upwards, is very modern, being subsequent to the super gypseous formation. I suppose it ought rather to be called a Porphyry; it is almost characterized by the absence of quartz. From the number of the points composed of this rock and their exact identity of nature, I cannot doubt but what they spring from one solid archway which follows the course of the mountains. If this view is correct; it would be surprising if metamorphic action was not visible in all the lower formations.
To return to the angular displacements; it is more rare to find horizontal than vertical strata, every where we see them dipping at high angles from certain lines of elevation.
This is common to the exterior, as well as central parts; the violence has however been much greater in the latter. The principal lines are roughly parallel, running North and South. but in the Alpine country on the west side, they are frequently oblique.
I have been able twice to prove and occasionally to offer presumtive [sic] evidence, that the strata of the exterior ridges have been inclined and broken subsequently to those of the grand central line. I believe this to
(14) Recapitulations &c
be a general occurrence. Whether the excessive violence, betrayed in any one ridge, has been the result of one or more paroxysms, I can bring forward no evidences. But I cannot refrain from remarking, that as the inclination of the strata is owing to the injection of a mass of fluid stone, I can see no reason, why fresh matter should not be pumped into its centre, which long would remain fluid, as, that reiterated eruptions should take place from the same volcano.
Looking at the history of the Cordilleras; we first see an epoch of excessive volcanic eruption which has never since been nearly equalled. Secondly there has been a long period of comparative quiescence, during which, the great masses of sediment of the two upper divisions, were precipitated and accumulated. Thirdly we again have a period of extreme volcanic energy; now however the fluid stone is injected and not erupted. Is this difference owing to the plain mechanical cause, of the increased incumbent pressure? This second general phase of activity has, like the first, happened beneath the waters of the sea. – I ground my conviction on the frequency, of the inclined strata, being covered by subaqueous deposits, horizontally stratified. – I may adduce the plains and basins (Uspallata 6 – 7000 feet) which are composed of stratified shingle and are often modelled into successive terraces. At Copiapo in both valleys, where there are uncommon scenes
(15) Recapitulations &c
of violence, I found a lofty plain of gravel, and on the very ridge of the main Cordillera an horizontal formation of tufa. The argument drawn from the unbroken slope of the terraces placed on each side of the valleys within the central ranges, and the known manner of elevation near the coast has with me great weight. I do not deny, but that some of the loftiest pinnacles, may have been raised above the level of the waters by an angular movement. Moreover I believe that lines of great fracture (or faults) have occasionally crossed the dry land and elevated land. – Such an hypothesis has been used, in explaining the singular valley in the ascent of the Portillo pass. But speaking in general terms I can feel little doubt, but that in the Cordillera the whole of the present elevation above the sea is owing to a gradual and horizontal upheaval. I conceive these views are in perfect conformity with the existence of those level remarkable basin, situated on the summit of the Andes, we may instance – Titicaca – Cuenca elevated 1350 toises – the valley of Quito from 1340 to 1490 toises – and the grand Mexican platform which between 19° and 24 1/2 ° of latitude, remains constantly at the height of 950 to 1200 toises (Humboldt, Personal Narrative, Vol. VI. Part II)
Are not like the strata in these plains horizontal?” Can they all be thought lacustrine?.–
I am not however unaware of the difficulties of the theory, which would attribute the effort of raising these stupendous mountains
(16) Recapitulations &c
(an effort to which the rupture of a few thousand feet of strata is as nothing) – to a slow, prolonged, and perhaps scarcely sensible movement. – It will be remembered, that in the second phase, the fluid stone was said to be injected, but in this third one of prolonged upheaval, the force perhaps has less frequently ruptured, than raised the strata in mass. The central and principal lines of fracture being strengthened by the injected masses of crystalline rocks, would less readily give way; hence any small angular disturbances, which might (from the stretching of the whole body) subsequently happen, would in all probability only effect the exterior and weaker flanks. In support of this view it may be remembered that modern Tertiary formations on the shores of the Pacifick have been upheaved by distinct lines of elevation.
I hope I shall not be misunderstood, to say that the angular displacements in all the ridges, took place at any one period of time.
What I mean alone to express is that each happened beneath the sea. Indeed so far from thinking them contemporaneous, I strongly suspect that during the Chilian earthquakes, the motion which proceeds from Seawards is caused by the incipient formation of mountain ridges. I am sure, no one who has felt a really severe shock, will object on the score of such a phenomenon being so violent – If there is any truth in this analogy, we must believe that during
(17) Recapitulations &c
each paroxysm of angular movement, the whole surrounding country is slightly raised. To follow the idea, one step farther, as the latter movement is so much the greater effort, we are led to believe that the angular displacements and injected rocks are mere accidents depending on the general vertical propulsion in mass. –
The distinction between these two kinds of movements appears to me, very important. – It explains the frequency in the other parts of the world of unconformable stratification. If in the general elevation of land, the angular had borne a large proportion to the gradual vertical, movements this fact could not so frequently have occurred. The above facts are important in another point of view, viz the configuration of the land. After the angular movements have impressed the first and grand outlines, the slow and progressive rise above the ocean will have exposed every part to its untiring action. In the section of a range of mountains, the highest and central lines will be seen to have all their basal and more extended parts protected by the outer ridges. – If however, we add to this theory, the subsequent and distinct formation of such outer ridges, we then indeed have in the action of the sea a most powerful agent.
With respect to the valleys, and especially to those of the lower country, in another part I have attempted with so much detail to show that the same element has been the efficient agent, that it would be superfluous again to discuss the subject. If these
(18) Recapitulations &c
views are correct, it almost follows as a consequence, and independent of the many reasons which have been adduced, that the beds of diluvium, which fill up such wide intervals between the mountains, are of marine origin. The subject of continental elevations being so intimately connected with the above questions, it will be readily granted to be one of high importance.
I have called the gradual rise an horizontal upheaval. – Speaking with accuracy, the more probable movement is that of a curved enlargement of a narrow space of the superficies of the globe. Hence the vertical rise in the centre will be much more then towards the outskirts. When we prove that within a certain period the coast has been elevated 300 feet, the Cordilleras may at the same time have attained an additional height of 6 or 900 feet. I can attribute to this cause alone, the extraordinary rapidity of the river courses throughout the whole of Chili. Although flowing through great valleys, which to the eye appear level, they may be described as broad mountain torrents. On this fact, so remarkable in the geography of Chili, depends the facility of irrigation and consequent fertility of that whole country.
The Maypo in a course of 60 miles, from its mouth (measuring the larger curvatures alone) must attain an elevation of 1600 feet.
omitted (a) (Mier’s Chili). All the rivers, when swollen by the waters of the melting snow rush along with extreme impetuosity, and bear to the
[18v]
(a) The river of Aconcagua, in a like manner in a course of nearly 50 miles, rises at Villa Nueva to a height of about 2500 feet. (Miers Chili).
(19) Recapitulations &c
sea large quantities of matter, torn from the sides and bottoms of their channels. That so great a slope should still, with this daily action exist, appears to me to be most easily explained by an accumulation of progressive elevation of the kind already indicated. It is stated that in the Valparaiso earthquake of 1822, the land at the distance of 5 or 6 miles inshore, rose in the proportion of three to one, to that on the coast. On the other hand, it must be confessed, than in 1835 at Concepcion, the rise of the Island of St. Mary was greater than on the neighbouring mainland.
That the continental elevation is a phenomenon intimately connected with the rise in Mass of the Andes, I think no one will dispute..
In Patagonia, within the recent period I have shown that its influence has been felt on the whole coast at the distance of 2 to 300 miles, and in the Province of Buenos Ayres at upwards of twice that distance, In Patagonia we have proofs that the coast has been elevated in a horizontal manner to a height of from 300 to 400 feet; and it may be assumed that there is no part which has not risen. On the Northern coast the rise has been little which perhaps is owing to the increased distance from the Andes. –
These same facts are applicable to the whole of Chili (and probably from some data to the whole of and certainly to a part of Peru) in the central divisions of Chili perhaps the rise has been rather greater than towards
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its extremities. The proofs have generally been drawn from shells lying unprotected on the surface of the ground. The duration of these shells, it must be remembered is limited, although I believe the period is by no means short. Therefore if one part of the land was raised a hundred feet some thousands year prior to the rest of the coast, it is possible no evidence of such elevation might now exist. – Notwithstanding this source of inaccuracy, I believe all the southern part of South America has been elevated within certain limits equably. It is in conformity with this view, that the severe earthquakes, which overthrow cities are believed both in Chili and Peru to recur after nearly regular intervals. – The two phenomena are so intimately connected, that if one is part of a regular series the other probably will obey similar laws.
Some arguments, drawn from the structure of the plains of Patagonia tell on the same side of the question.
There exists also a negative evidence, that no district has been raised, far above the usual range; namely that some extinct species of shells are commonly found imbedded in the cliffs of the sea coast along both shores of the continent. Now without we suppose a period of general depression; it is clear that these strata have only been raised those number of feet, at which depth the shells could have lived. – Suppose this depth one
(21) Recapitulations &c
thousand feet. Then as some of the species are extinct, the elevation of the land, near the present shore, could not within the Recent period have equalled one thousand feet. I must again urge, that this does not apply to the Cordillera. Indeed if it did apply to that range of mountains and we were obliged to take the rise on the coast as our analogy for the whole elevation – how immense must be the interval of time between the deposition of the littoral Tertiary and the super gypseous formations. But as we have shown, that there have been epochs of Volcanic energy differing in intensity it is impossible, to feel sure whether such rise was more or less rapid formily [sic] than at present.
I have only mentioned this recent elevation of those coasts which I have myself visited namely La Plata. Patagonia. Tierra del Fuego. Chili and Peru. The following Notes have been drawn from various authorities; they tend to show that the whole continent of the two Americas has been subjected to the same grand movement
( &c &c &c &c )
These continental elevations are very extraordinary phenomena. – Seeing such plains, as those of the Pampas, raised with so equable a force, as not even to destroy the original slope of their deposition; one is tempted to conclude, that nothing, but the expansion from heat of a solid inferior mass, could effect this. –
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I should rather however suspect a gradual movement of the internal fluid mass towards that part of the superficies of the world. – These plains which south of Latitude 30° fringe the eastern base of the Andes, form a prominent feature in the geography of South America. Has the direction of the currents allowed the deposits, proceeding from the mountains tranquilly to accumulate on one side alone? Have not such accumulations been favoured by the basin or trough formed by the Porphyries of Patagonia, and the granites of La Plata and Brazil? But I think we must look for a more efficient cause in a strength of elevation greater and more widely extended on one than the other side; it is part of the same great cause which has developed both in North and South America the great eastern protuberances, we may also notice (and perhaps there may be some connection between these facts) that throughout the southern part of the Cordillera the smaller secondary ranges appear to lie chiefly on the side of the Pacifick. On the opposite side, this absence of such inferior lines of elevation, is accompanied by the fact, long since remarked, of an extreme infrequency of active Volcanos. In the only three spots, where I visited the eastern flank (Uspallata. Portillo. and S. Cruz) there were subaqueous volcanic formations. In the first mentioned place, the fact is not strong,
(23) Recapitulations &c
because the streams belong to one, although the latest of the regular formations. But in the two latter, they are disconnected and of posterior age. Now these all being subaqueous, we may imagine we see an example of the common law, that eruptions only take place near to the sea-coast. At the present day, not only are there not Volcanos, but even the Earthquakes of the shores of the Pacifick are felt on the eastern slope with much moderated violence.
Humboldt has remarked it is very rare to meet within like Cordilleras any Lava streams. In my limited examination, I only met with one mass of lava, (and that of small extent at Cauquenes) which could probably have flowed as a subaerial stream. – Excepting some bits of pumice imbedded in the superficial deposit on the plains of St. Jago, and a little Trachyte near Copiapo I did not even see any loose fragments of such formations. – Yet within the central Cordillera there are several inert volcanos, – indeed I suspect, all the groups of the highest peaks, which rise far above the solid wall, are remains of craters. –
The peak of Aconcagua, I have been assured is a volcano, and has been seen in semi-action. When its stupendous height (2300 feet) is considered, and the depth of the neighbouring Pacifick ocean is taken into consideration, we feel
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astonished that any part of the crust of the globe can withstand the pressure of a column of four or five miles high of liquid lava. The volcanos are situated in the central parts of the Andes, and all, as I believe, pour forth trachytic lavas alone; indeed I have scarcely seen even a fragment of true basalt in Chili. From reasons, already assigned, the volcanic orifices must all conduct to the great archway of syenitic greenstone. This hypothetical connection, reminds one of a similar but real one observed in the trachytes of Mexico. Hungary and the Rhine. – It is certain, that since the Cordillera have been raised into dry land, volcanic eruptions, neither have played, nor do play any but a very subordinate part. Formerly we looked at the injection of semi-liquified matter, beneath the lines of fractured strata, as an accident depending on the grand rise in mass. May we not, in a like manner consider these volcanos, as mere accidents, consequent on a more complete rupture of the strata, and perhaps connected with the contact of heated with humid rocks, – accidents which happened during the periods, when that far more important phenomenon of injection determined the lines of elevation
(a) The very frequent occurrence of metallic veins in the mountains of the Andes, is too general a fact to be overlooked; as far as I have been able to observe they
[24v]
Note (a) As in these mountains the lines of elevation are supposed to have uplifted the rocks, when beneath the waters of the sea, so, according to the above conjectures, the volcanic mouths, probably originated under similar circumstances. Are not the ascertained cases, of a line of elevation crossing any terrestrial district, or of a volcano quite bursting out in such a situation unconnected with any previously existing focus, either deficient or excessively rare? If these phenomena are connected and generally happen beneath the sea, we must wait long, before such instances can be recorded. –
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are found in all the five classes or formation. – Perhaps Copper and Gold more affect the lower crystalline rocks, whilst Silver is found in the upper stratified series.
hence mines of the former metals are generally situated nearer to the coast, than those of Silver, I noticed this in a map, where with the mines, the kind of metal was marked down. – The Veins abound in certain circumscribed spaces. (“Minerales”), and are not scattered indiscriminately over the country. –
I suspect the most favourable points, are where the strata have undergone most violence, as shown by the numerous stony dikes, the altered mineralogical nature, and disturbed stratification. This more especially appears to be the case, where the injected masses of crystalline and formerly melted rocks have reached the higher series. – It frequently happens that the mine is worked from the mantle composed of such strata to the underlying mass. In these instances, I cannot avoid supposing that the metals have been sublimed from below. – On the other hand, from the structure of the veins, in some of the silver “minerales”, strong arguments, might I suspect, be drawn to show, that the metallic substances are there of contemporaneous origin. – The Chileno Miner, puts great faith in the colour and appearance of the ground (which he calls Paniza) for producing mines. – On seeing distant mountains of peculiar colours, he will exclaim, “this is a good Paniza for silver and that for gold.” It is said also, that the
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richest part of the vein lies in the thickest part of the mountain. – With some exceptions, the veins of all the metals, have one general direction, which is about N N W and S S E. –
Where they vary, the tendency is towards N W and S E. – These directions are common likewise to the clearage [cleavage] of the Slates. – The veins are very frequently crossed by dikes, which consist of many kinds of very compact rocks.
These dikes, from their position, must have been injected at a very remote period, since which time, the form of the land has been greatly altered; I have always felt inclined to attribute their origin, in each case to the earlier epochs of disturbance, when that line of mountain was most violently shaken. If this is so, generally the metallic veins must be of an equal, or rather of an earlier date. Yet, as compared to the formations of the world the period cannot be one of very high antiquity. –
(a) Every body must have been struck with the Andes, as forming so prominent a feature on the surface of the Globe. – Its true dimensions have not, I think, been always, fully understood – From seeing maps, drawn on a small scale, one is tempted to consider the whole as one ridge instead of many parallel ranges of mountains. – The general breadth of the mass varies in a transverse section (according to Humboldt) from 18 to 22 leagues. The Alpine country, which separates the central lines from the Pacifick, is in fact part of the Cordillera; there is seldom any natural division
[26v]
Note (a)
For an admirable and detailed description of the Cordillera of the Andes, see the Personal Narrative, Vol = VI. Part II.
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between them. – Taking this also into consideration, the Andes may be described as a track of mountainous country exceeding in breadth the northern part of the peninsula of Italy. – yearly its highest pinnacles are recognized as being more lofty – Aconcaqua, is now known to exceed in height Chimborazo and almost to equal Sorata. The lowest gap in this great barrier in central Chili, has an altitude of upwards of 10,000 feet. – To form a true estimate of these heights, we must also take into consideration the great depth of the neighbouring Pacifick ocean.
We have shown that all the matter, of which these great ridges are composed, may be considered as directly or indirectly emptied from the bowels of the earth. – Stupendous as the mass is, I believe it is inconsiderable as compared to the detritus, which has accumulated during the lapse of ages at the basal plains. – It must be remembered, that these plains on the eastern flank are from 3000 to 4000 feet high, (equals to Vesuvius!) and that from this elevation they slope through a space of 500 miles, to the shores of the Atlantic, or at a nearer line are intersected by cliffs of 200 to 400 feet in height. – To this vast sheet of matter we must add at least an equal amount, which lies buried in the depths of the Pacifick.–
Nor when considering the mass of the Andes must we forget the Mexican platform, a feature in size scarcely paralleled in the world. The great historian of South America
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says, “To give a just idea of the colossal dimensions of the Andes, I shall here observe that the surface of the lake of Titicaca alone (448 square marine leagues) exceeds twenty times that of the lake of Geneva, and twice the mean extent of a department of France.” The extreme length of the Cordillera is familiar to the mind; it may be considered as a linear train of ancient and modern points of eruption. – I have shown that such are found in Tierra del Fuego; from this point to Mount St. Eelias, (situated in the Cordillera of the coast) we have a nearly unbroken chain extending through 116 degrees of Latitude. One is strongly tempted to add, as part of the Cordillera, the volcanos of South Shetland (7 degrees nearer to the antarctic pole) which fall in the line of curvature of the outer coast of Tierra del Fuego. – The majestic grandeur of the Cordillera of the Andes, is augmented in our imaginations, when reflect, that the great subterranean movements, to which their origin must be attributed, have continued in activity, from the epoch of Ammonites, to the present day; – That now the Volcano spouts out its fiery contents, and the solid land rises in mass! –
To the view of a Geologist, besides this grandeur in dimensions and the antiquity of origin, there is a simplicity which is very striking. – We see, a symmetrical chain of mountains, rises through a granitic platform, which latter of enormous extent, composes on one side, the shores of the Pacifick and on the other, the
(29) Recapitulations &c
grand united basins of La Plata, the Amazons and the Orinoco. – The Andes and their accumulated detritus, together with this one great bed of crystalline rocks compose South America. – The whole has been raised from beneath the ocean into dry land by the action of one connected force.
When we compare this Continent to the small one of Europe, we are astonished to see in the latter, a surface varied by bands and remnants of formations, belonging to all ages, from the oldest to the most recent. The mountain chains follow independent directions. – The Volcanos are not connected on any one line of fissure and their efforts have been insignificant. There are signs of repeated elevations and depressions. – Some can see the tracts of overwhelming deluges. – The outline of the land is intersected by the waters of the sea and its surface divided into an infinity of drainage basins. – on the other hand in laying the foundation of South America, Nature chose a simple plan – but upon that basis she has raised a magnificent structure!