The Elements of Geology eBook

This eBook from the Gutenberg Project consists of approximately 328 pages of information about The Elements of Geology.

Over a long belt which reaches from Wyoming across Colorado into New Mexico no Triassic sediments are found, nor is there any evidence that they were ever present; hence this area was high land suffering erosion during the Triassic.  On each side of it, in eastern Colorado and about the Black Hills, in western Texas, in Utah, over the site of the Wasatch Mountains, and southward into Arizona over the plateaus trenched by the Colorado River, are large areas of Triassic rocks, sandstones chiefly, with some rock salt and gypsum.  Fossils are very rare and none of them marine.  Here, then, lay broad shallow lakes often salt, and warped basins, in which the waste of the adjacent uplands gathered.  To this system belong the sandstones of the Garden of the Gods in Colorado, which later earth movements have upturned with the uplifted mountain flanks.

The Jurassic was marked with varied oscillations and wide changes in the outline of sea and land.

Jurassic shales of immense thickness—­now metamorphosed into slates—­are found infolded into the Sierra Nevada Mountains.  Hence during Jurassic times the Sierra trough continued to subside, and enormous deposits of mud were washed into it from the land lying to the east.  Contemporaneous lava flows interbedded with the strata show that volcanic action accompanied the downwarp, and that molten rock was driven upward through fissures in the crust and outspread over the sea floor in sheets of lava.

The Sierra deformation.  Ever since the middle of the Silurian, the Sierra trough had been sinking, though no doubt with halts and interruptions, until it contained nearly twenty-five thousand feet of sediment.  At the close of the Jurassic it yielded to lateral pressure and the vast pile of strata was crumpled and upheaved into towering mountains.  The Mesozoic muds were hardened and squeezed into slates.  The rocks were wrenched and broken, and underground waters began the work of filling their fissures with gold-bearing quartz, which was yet to wait millions of years before the arrival of man to mine it.  Immense bodies of molten rock were intruded into the crust as it suffered deformation, and these appear in the large areas of granite which the later denudation of the range has brought to light.

The same movements probably uplifted the rocks of the Coast Range in a chain of islands.  The whole western part of the continent was raised and its seas and lakes were for the most part drained away.

The British Isles.  The Triassic strata of the British Isles are continental, and include breccia beds of cemented talus, deposits of salt and gypsum, and sandstones whose rounded and polished grains are those of the wind-blown sands of deserts.  In Triassic times the British Isles were part of a desert extending over much of northwestern Europe.


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The Elements of Geology from Project Gutenberg. Public domain.
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