124 J. LE CONTE — EARTH- CRUST MOVEMENTS AND THEIR CAUSES. 



elevation in this case is associated with, and in spite of, loading. The 

 elevation, as we all know, commenced in late Tertiary and culminated 

 in early Glacial. This elevation was, at least, one cause, probably the 

 main cause, of the cold and the ice accumulation, but the elevation con- 

 tinued in spite of the accumulating load of ice. Finalty, however, the 

 accumulating load prevailed over the elevating force and the previously 

 rising area began to sink, but only because the interior elevatory forces 

 had commenced to die out. Then with the sinking commenced a modera- 

 tion of the climate, melting of the ice, removal of load, and consequent 

 rising of the crust to the present condition, but far below the previous 

 elevated condition, because the elevating forces, whatever these were, 

 had in the meantime exhausted themselves. If it had not been for the 

 interference of the ice-load, I suppose that instead of the double oscilla- 

 tion which actually occurred there would have been a simple curve of 

 elevation coming down again to the present condition, but culminating 

 a little later and rising a little higher than we actually find it did. 



The question arises as to how great an area is necessary for the opera- 

 tion of the principle of isostasy ? What extent and degree of inequality 

 of surface may be upheld by earth rigidity alone ? 



The recent transcontinental gravitation-determinations by Putnam 

 and their interpretation by Gilbert* seem to show a degree of rigidity 

 greater than previously supposed. They seem to show that while the 

 whole continental arch is certainly sustained b} r isostasy — that is, by 

 deficiency of density below the sea level in that part, the continental 

 area being lighter in proportion as it is higher — yet great mountain 

 ranges like the Appalachian, Colorado, and Wasatch mountains show 

 no such means of support, but are bodily upheld by earth rigidity ; and 

 even great plateaus like the Colorado plateau, 275 miles across, are largely, 

 though not entirely, sustained in the same way. 



Monoclinal Mountain Ranges. 



Until recently mountain ranges were supposed to be all made in one 

 way, namely, by lateral crushing and strata-folding and bulging along 

 the line of yielding. To Gilbert is due the credit of having first drawn 

 attention to another type, conspicuously represented only in the Plateau 

 and Basin region, especially the latter — that is, those produced by tilting 

 and irregular settling of the crust blocks between great fissures. The 

 two types of mountains are completely contrasted in all respects. As to 



* Gilbert : Phil. Soc. Washington, vol. 13, 1895, p. 31. Gilbert : Jour. Geology, vol. 3, 1895, p. 331. 

 O. Fisher : Nature, vol. 52, 1895, p. 433. 



