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Transportal of Gravel

Considering the shingle plains of Patagonia, in connection with the somewhat similar facts noticed in Chili & Chiloe, I am strongly urged to believe some general cause, & not any sudden period of violence, has spread been the agent in spreading gravel over the tertiary formations. In the first place I do not believe the sea has the power of transporting gravel to any distance over its bottom, but on the other hand, near the coast, where the inclination is greater, action of tides & surf during gales may in all probability be able to effect this. —

not boulders ∴ not ice
not rivers

Again in all these step-formed plains it is clear each step must be thus modelled in a shallow sea near the coast. — In the plain of Patagonia I by thought it necessary to believe that the whole seas bottom had primarily been coated with gravel & this subsequently remodelled. — [illeg words in margin] Here there was the difficulty what power had transported so many miles such immense quantities of gravel. —

Let us try another hypothesis. —

During the long succession of years, when sediment was gradually forming at the bottom of the S. Atlantic ocean a great mass of beds, an enormous accumulation of gravel must have been piled up at the Eastern submarine foot of the Andes. — When the land elevations took place, & one of the first or highest plains took place was found, the remodelling action of the shoal & powerful sea would remove the finer sediment & leave only a thick bed of gravel; when, this other plane was exposed to in its turn, part of

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1.

[sketch] Andes Z level of ocean gravel fine sediment

2.

[sketch] Andes Z A B fine sediment level of ocean x

there not

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S. Cruz

it would be destroyed & the gravel spread out in a thinner layer over another plain (or for seas bottom). — this process being repeated, the gravel would at length be carried far to sea-ward by the action of the sea near to a beach; but necessarily the thickness & quantity must be greatly diminished in each time successive plain. —

The Diagram, will show what I mean. No. 1 represent what is supposed to have been originally the case; No. 2 after some elevations Z.X being the bottom of the Atlantic.

No. 2. after some elevations. the cliff AE would be formed & the bottom ZX would have been a different shape; hence the mass of gravel included between ABCE has been removed & would be remodelled over the bottom as far as tidal power could carry it: Part of this after another elevation, would, as represented, be again spread out in a thinner sheet, & so on ad infinitum; the finer particles always being removed to a greater distance. — Now at S. Cruz, at the coast the gravel is not above 50 feet thick, & at 100 miles inland it is 212 ft thick. — Against this hypothesis I have three objections

it leaves unaccounted the transportal of the enormous angular blocks of ancient rocks; the facts, which I have adduced to show how little motion gravel has ever in shoal, turbulent sea. — & lastly, the appearance of the pebbles having come from the northward. (It might be conjectured in answer to this last objection, there are eddy streams swept by the foot of Andes to the South (a), owing to a, current

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(a) such, as now exist, sweeping round. C. Horn. ) —

My mind is I cannot make up my mind on this question, but I strongly suspect there is a constant relation between the spreading out of gravel & that part of the ocean bottom having been elevated to within a moderate distance of the coast line.—

؟ The change of pebbles on plain from those in the formation owing to changes in the rock of the Andes itself?

The great boulder at Geneva is 10 feet high & 20 x 15 ft [c. 4 words illeg]¹

Some of the pebbles must have proceeded from the Port Desire porphyrys.

The same force which carried the angular blocks. might have transported the famous ones. from central Alps to Jura. only 50 miles. — The mass of shingle. said to be caused by violence after formation of the main valleys De La Beche Saussure — mark of aqueous corrosion 200 toise above present level of the Rhone — blocks being placed in groups & lines — convince me work of sea — did Jura rise with the blocks, as Cordilleras of Copiapò with the horizontal tufa 10,000 ft! — if this latter could remain, so could blow [illeg] Hence the form valley of Geneva. —

NB. The alternate bands of boulders &c &c East side of Andes of St. Jago show no debacles had passed down through vallies: certainly blocks on plains are not seen then. because smooth over with lacustine deposits. — but argument should be extended

Range of Boulders in valleys of Northern Chili

¹ An erratic block. Playfair 1802 vol. 1 p. 387: "One of the largest blocks of granite that we know of, is on the east side of the lake of Geneva, called Pierre de Gouté, about ten feet in height, with a horizontal section of fifteen by twenty." Darwin mentioned the same block in his Galapagos notebook.