In the recent considerations of subaërial formations so many novelties enter that in many an old and well known field a new interest is aroused. Prominent among such tracts is the country lying between the Rocky Mountains and the Mississippi River. Both for the origin of the vast plains surface and of the so-called fresh-water Tertiaries underlying it a more satisfactory explanation than any vet offered is now demanded.

The Great Plains appear to display the effects of a general leveling process to which but scanty attention has been given. On a grand scale they seem to introduce to us a mode of terranal genesis hitherto almost unrecognized. Continental deposits thus begin to assume in this country an importance which has been never before accorded them.

The vast areal extent of the Great Plains terranes, their remarkable uniformity in lithologic character, their unusual evenness or vagueness of stratification, and . . .

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