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Updated: May 4, 2025
No doubt it is now in progress where deep-buried sediments are invaded by heat either from intrusive igneous masses or from the earth's interior, or are suffering slow deformation under the thrust of mountain-making forces. Suggest how rocks now in process of metamorphism may sometimes be exposed to view. Why do metamorphic rocks appear on the surface to-day?
But, by rubbing up some chalk with a brush in water and then pouring off the milky fluid, so as to obtain sediments of different degrees of fineness, the granules and the minute rounded bodies may be pretty well separated from one another, and submitted to microscopic examination, either as opaque or as transparent objects.
But sediments from the land are spread within a zone but two or three hundred miles in width along the margin of the continents, a line one hundred thousand miles long.
The facts above presented will show the reader that the marine sediments are formed under conditions which permit a great variety in the nature of the materials of which they are composed. As soon as the deposits are built into rocks and covered by later accumulations, their materials enter the laboratory of the under earth, where they are subjected to progressive changes.
The stratified rocks of the land are for the most part ancient deposits of the sea, which have been lifted above sea level; and we may believe that the sediments now being laid offshore are the "dust of continents to be," and will some time emerge to form additions to the land.
Between the layers are found in places old soil beds and forest grounds and the sediments of lakes. Hence the interval between the flows in any locality was sometimes long enough for clays to gather in the lakes which filled depressions in the surface.
If the flow occurred from some submarine volcano, we may recognize the fact by the sea-laid sediments which cover it, filling the cracks and crevices of its upper surface and containing pieces of lava washed from it in their basal layers.
We have already noted the fact that the continents are old, which implies that the parts of the earth which they occupy have long been the seats of tolerably continuous erosion. Now and then they have swung down partly beneath the sea, and during their submersion they received a share of sediments.
The long belt lying west of the ancient axes of the Colorado Islands and east of the Great Basin land had been an area of deposition for many ages, and in its subsiding troughs Paleozoic and Mesozoic sediments had gathered to the depth of many thousand feet. And now from Mexico well-nigh to the Arctic Ocean this belt yielded to lateral pressure.
In nature, shells and other relics of animals or plants are often removed by being dissolved by percolating waters, and the molds are either filled with sediments or with minerals deposited from solution. Interior casts of shells reproduce any markings on the inside of the valves, and casts of the interior of the skulls of ancient vertebrates show the form and size of their brains.
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