Page:Popular Science Monthly Volume 39.djvu/694

676 is conspicuously seen from the Northern Pacific Railroad. These mountains trend slightly west of north, and extend about forty miles with a width of fifteen miles, attaining an elevation of 11,178 feet above the sea and 5,000 to 6,000 feet above the prairies at their base. Their structure has been thoroughly studied by Wolff, who finds that they consist of late Cretaceous strata, soft sandstones, nearly horizontal in stratification, intersected by a network of eruptive dikes. The more enduring igneous rocks have preserved this range, while an average denudation of not less than one mile in vertical amount reduced all the surrounding country to a base level of erosion. The Highwood Mountains, about 25 miles east of Great Falls, Montana, having a height of 7,G00 feet above the sea, or about 3,500 feet above their base, are described by Davis as displaying the same structure, and therefore similarly testifying of great denudation. This erosion of the Great Plains was probably in progress during the whole Tertiary era. Around Turtle Mountain, on the boundary between North Dakota and Manitoba, its amount was not less than 500 to 1,000 feet.

Original epirogenic uplifting of these plains took place at the end of the Cretaceous period, or during the early part of the Eocene. Thence onward through the Tertiary era, rains, creeks, and rivers were reducing this region nearly to the sea-level, excepting remnants like the Crazy, Highwood, and Turtle Mountains, which were being sculptured approximately to their present form. But the Tertiary era seems to have been terminated and the Quaternary ushered in by a new epirogenic differential uplifting of this continent, causing the accumulation of the ice-sheet of the first Glacial epoch. The time of great elevation initiating the Ice age, and the ensuing long interglacial epoch before the second glaciation, appear to have permitted rivers in North Dakota and Manitoba to wear away a considerable part of the Tertiary base-leveled plain, from its former eastern margin to the remarkable escarpment, in part a small eroded mountain range, of the Pembina, Riding, and Duck Mountains and the Porcupine and Pasquia Hills, which form the west border of the Red River Valley plain and of the lowland with large lakes in central Manitoba.

Reviewing this classification of mountain ranges for the purpose of discovering what elements of diversity and of unity characterize the manifestations of mountain-building energy, we see this to be of two kinds, the second being presented under four phases. The first kind of mountain-building energy, producing folds, is' evidently lateral pressure, and is ascribed by geologists and physicists to the contraction of the earth's mass by its secular cooling, with resulting adaptation of the rigid outer part of the crust to the shrinking interior. The second is energy acting