CLASSIFICATION OF MOUNTAIN RANGES. 677 



tically upward, which, has produced the four other types of con- 

 structive mountain ranges and masses by diverse phases of its 

 manifestation namely, the slow arching of limited areas, as the 

 Uinta, Junction, and Yampa Mountains; the sudden volcanic 

 lifting of the laccolite mountains ; the upheaval and subsidence, 

 with faulting and tilting, of the Basin ranges ; and the outpour- 

 ing of lava, as in the Cascade Range. Each of these four phases 

 of vertically acting energy depends upon a viscous and plastic 

 (neither solid nor perfectly liquid) condition of the earth's interior. 

 Greater pressure of some portions of the crust than of others upon 

 the plastic interior would induce each phase of upward energy in 

 mountain-building. Where isolated blocks of the crust yielded 

 slowly to the resulting quasi-hydrostatic pressure of the interior, 

 mountains of the Uinta type were formed ; but large areas, as the 

 Great Basin, being swelled upward and anon subsiding, as the in- 

 terior pressure increased and diminished, have become marked by 

 tilted mountain ranges. Where the relations of intense heat, im- 

 mense pressure, and chemical influences, with presence of water 

 or its further ingress, have allowed portions of the interior, often 

 of great extent, to become liquid lava, its extravasation by the 

 same pressure has formed laccolite mountains and erupted mount- 

 ain masses, while many volcanic cones have been mainly built up 

 of fragments of solidified lava, much of it so fine as to be called 

 ashes, explosively ejected. 



In an appendix of Wright's Ice Age in North America, I have 

 pointed out the source of the relationship by which these two 

 kinds of mountain-building energy are united, both being caused 

 by the earth's contraction in cooling, and the second or upwardly 

 acting kind of energy being dependent on the first in the inter- 

 mittent and occasional relief of stress of the earth's crust by its 

 folding along the great orographic belts. Between the epochs of 

 mountain-building by plication, the diminution of the earth's 

 mass produces epirogenic distortion of the crust, by the elevation 

 of certain large areas and the depression of others, with resulting 

 inequalities of pressure upon different portions of the interior ; 

 and these effects have been greatest immediately before relief has 

 been given by the formation of folded mountain ranges. There 

 have been two epochs pre-eminently distinguished by extensive 

 mountain- plication, one occurring at the close of the Palaeozoic 

 era and another progressing through the Tertiary and culminat- 

 ing at the beginning of the Quaternary era, introducing the Ice 

 age. During the last, besides plication of the Coast Range, of the 

 Alps, and the Himalayas, a very extraordinary development of 

 tilted mountain ranges, and outpouring of lavas on an almost un- 

 precedented scale, have taken place .in the Great Basin and the 

 region crossed by the Snake and Columbia Rivers. With the cul- 



