676 THE POPULAR SCIENCE MONTHLY. 



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 testify- 

 ing 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, ex- 

 cepting remnants like the Crazy, Highwood, and Turtle Mount- 

 ains, 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 up- 

 lifting 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 pur- 

 pose of discovering what elements of diversity and of unity char- 

 acterize 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 ver- 



