Page:The Journal of geology (IA journalofgeology11893univers).pdf/133

This page has been proofread, but needs to be validated.
ORIGIN OF MOUNTAIN RANGES.
559

of the present Appalachian crest. The sediments along this marginal sea-bottom increased in thickness during Cambrian, Silurian, Devonian and Carboniferous (with some changes of Physical Geography, but without greatly changing the line of sedimentation) until 40,000 feet thickness was reached. Such thickness, of course, could not be attained without pari passu subsidence. We have additional evidence of this in shallow water fossils and even shore marks at many levels in the series. At the end of the coal period, when 40,000 feet had accumulated, the increasing softening along the line caused it finally to yield to horizontal thrust: the whole mass of strata was crumpled together and swelled up along the line of sedimentation and the Appalachian Range was born. The same forces which caused its birth continued to cause its growth for a long time. Subsequent erosion has sculptured it into its present form, but has not exposed its granite core. The crest is on the east or landward side, as we should expect, and the overfolds are to the west or toward the sea of that time. This is perhaps the most typical example we have.

2. Sierra.—If it were not for a subsequent movement so late as the beginning of the Quarternary, which greatly modified its form, the Sierra too would be a typical range. During the whole Palæozoic and the greater part of the Mesozoic the place now occupied by the Sierra was the eastern marginal bottom of the Pacific, receiving sediments from a continental land-mass in the present Basin region. The shore line changed somewhat at the end of the Palæozoic, but the Sierra region maintained a sea bottom position. At the end of the Jura, when an enormous thickness had accumulated, the increasing softening of the crust determined a yielding to lateral thrust and consequent formation of the range. Subsequent erosion has completely removed the strata from the crest and exposed the granitic core as an axis[1]. This axis is here also on the landward side, and the overfolds are

  1. ↑ Sierra granite is not Archæan as has been asserted by some, nor does it all antedate the birth of the range. This is proved (1) by the gradation traceable between slates and granites, and (2) by the fact stated by Whitney, by Fairbanks, and by Diller—that the granite in many places penetrates the slate as veins.
This article is issued from Wikisource. The text is available under Creative Commons Attribution-Share Alike 4.0 unless otherwise noted. Additional terms may apply for the media files.