第18章 ON THE ELEVATION OF THE EASTERN COAST OF SOUTH AME

The transportal and origin of this vast bed of pebbles is an interesting problem.From the manner in which they cap the step-formed plains, worn by the sea within the period of existing shells, their deposition, at least on the plains up to a height of 400 feet, must have been a recent geological event.From the form of the continent, we may feel sure that they have come from the westward, probably, in chief part from the Cordillera, but, perhaps, partly from unknown rocky ridges in the central districts of Patagonia.That the pebbles have not been transported by rivers, from the interior towards the coast, we may conclude from the fewness and smallness of the streams of Patagonia: moreover, in the case of the one great and rapid river of Santa Cruz, we have good evidence that its transporting power is very trifling.This river is from two to three hundred yards in width, about seventeen feet deep in its middle, and runs with a singular degree of uniformity five knots an hour, with no lakes and scarcely any still reaches: nevertheless, to give one instance of its small transporting power, upon careful examination, pebbles of compact basalt could not be found in the bed of the river at a greater distance than ten miles below the point where the stream rushes over the debris of the great basaltic cliffs forming its shore: fragments of the CELLULAR varieties have been washed down twice or thrice as far.That the pebbles in Central and Northern Patagonia have not been transported by ice-agency, as seems to have been the case to a considerable extent farther south, and likewise in the northern hemisphere, we may conclude, from the absence of all angular fragments in the gravel, and from the complete contrast in many other respects between the shingle and neighbouring boulder formation.

Looking to the gravel on any one of the step-formed plains, I cannot doubt, from the several reasons assigned in this chapter, that it has been spread out and leveled by the long-continued action of the sea, probably during the slow rise of the land.The smooth and perfectly rounded condition of the innumerable pebbles alone would prove long-continued action.But how the whole mass of shingle on the coast-plains has been transported from the mountains of the interior, is another and more difficult question.The following considerations, however, show that the sea by its ordinary action has considerable power in distributing pebbles.Table 3 above shows how very uniformly and gradually the pebbles decrease in size with the gradually seaward increasing depth and distance.(I may mention, that at the distance of 150 miles from the Patagonian shore I carefully examined the minute rounded particles in the sand, and found them to be fusible like the porphyries of the great shingle bed.I could even distinguish particles of the gallstone-yellow porphyry.It was interesting to notice how gradually the particles of white quartz increased, as we approached the Falkland Islands, which are thus constituted.In the whole line of soundings between these islands and the coast of Patagonia dead or living organic remains were most rare.On the relations between the depth of water and the nature of the bottom, see Martin White on "Soundings in the Channel" pages 4, 6, 175; also Captain Beechey's "Voyage to the Pacific"chapter 18.) A series of this kind irresistibly leads to the conclusion, that the sea has the power of sifting and distributing the loose matter on its bottom.According to Martin White, the bed of the British Channel is disturbed during gales at depths of sixty-three and sixty-seven fathoms, and at thirty fathoms, shingle and fragments of shells are often deposited, afterwards to be carried away again.("Soundings in the Channel" pages 4, 166.M.Siau states ("Edinburgh New Philosophical Journal" volume 31 page 246), that he found the sediment, at a depth of 188 metres, arranged in ripples of different degrees of fineness.There are some excellent discussions on this and allied subjects in Sir H.De la Beche's "Theoretical Researches.") Groundswells, which are believed to be caused by distant gales, seem especially to affect the bottom: at such times, according to Sir R.Schomburgk, the sea to a great distance round the West Indian Islands, at depths from five to fifteen fathoms, becomes discoloured, and even the anchors of vessels have been moved.("Journal of Royal Geographical Society" volume 5 page 25.It appears from Mr.Scott Russell's investigations (see Mr.Murchison's "Anniversary Address Geological Society" 1843 page 40), that in waves of translation the motion of the particles of water is nearly as great at the bottom as at the top.)There are, however, some difficulties in understanding how the sea can transport pebbles lying at the bottom, for, from experiments instituted on the power of running water, it would appear that the currents of the sea have not sufficient velocity to move stones of even moderate size: