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Updated: May 4, 2025
In a series whose basins lie in the same canyon, and are fed by one and the same main stream, the uppermost will, of course, vanish first unless some other lake-filling agent comes in to modify the result; because at first it receives nearly all of the sediments that the stream brings down, only the finest of the mud-particles being carried through the highest of the series to the next below.
Take the case to which I have previously referred. The finer or coarser sediments that are carried down by the current of the river, will only be carried out a certain distance, and eventually, as we have already seen, on reaching the stiller part of the ocean, will be deposited at the bottom.
If after studying the erosive phenomena exhibited in the structure of the earth the student takes up the study of the accumulations of strata, and endeavours to determine the time required for the laying down of the sediments, he finds similar evidence of the earth's great antiquity.
Even the saline springs of Canada and the Northern States of America, where the wapiti love to congregate, and the noble hunter lurks in the thicket to murder them unperceived, derive their saltness, as an able Canadian geologist has shown, from the thinly scattered salts still retained among the sediments of that very archaic sea whose precipitates form the earliest known life-bearing rocks.
THE SEDIMENTARY HISTORY OF FOLDED MOUNTAINS. We may mention here some of the conditions which have commonly been antecedent to great foldings of the crust. Mountain ranges are made of belts of enormously and exceptionally thick sediments. The strata of the Appalachians are thirty thousand feet thick, while the same formations thin out to five thousand feet in the Mississippi valley.
How rapidly a lake may be silted up under exceptionally favorable conditions is illustrated by the fact that over the bottom of the artificial lake, of thirty-five square miles, formed behind the great dam across the Colorado River at Austin, Texas, sediments thirty-nine feet deep gathered in seven years.
In the sea the conditions for preservation are more favorable than on land; yet even here the proportion of animals and plants whose hard parts are fossilized is very small compared with those which either totally decay before they are buried in slowly accumulating sediments or are ground to powder by waves and currents.
Take the case to which I have previously referred. The finer or coarser sediments that are carried down by the current of the river, will only be carried out a certain distance, and eventually, as we have already seen, on reaching the stiller part of the ocean, will be deposited at the bottom.
These sediments were afterwards converted into the first rocks of the so-called stratified or sedimentary series, as contrasted with the crystalline or plutonic rocks like the original mass of the earth and the kinds forced to the surface by volcanic eruptions.
In the Cordillera, I estimated one mass of conglomerate at ten thousand feet; and although conglomerates have probably been accumulated at a quicker rate than finer sediments, yet from being formed of worn and rounded pebbles, each of which bears the stamp of time, they are good to show how slowly the mass must have been heaped together.
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