448 REID— ISOSTASY AND MOUNTAIN RANGES. [Apni 21, 



that the earth is not strong enongh to sustain an added thickness of 

 more than alxtut two hun(h-ed and fifty feet of rock over an area as 

 large as a square degree without slowly yielding. The stations 

 where the ohservations were made are scattered over various parts 

 of this country, on the eastern coast, in the Appalachian mountain 

 range, in the region of the Great Lakes, near the Gulf of Mexico, 

 in the great plains of the Mississippi basin, on the great elevations 

 of the Rocky Mountains, the plateaux of Utah, the Sierra Nevada 

 mountains and the Pacific coast, regions exhibiting a great variety 

 of topographic forms and differing greatly as to geologic activity. 

 Whatever movements may be going on in the Rocky mountains, 

 and in the region between them and the Atlantic ocean, are certainly 

 very small ; whereas to the west, and particularly in the state of 

 California, the movements seem to be very active. The eastern 

 edsfe of the Sierra Nevada received additional elevation at the time 

 of the Owens Valley earth(iuake in 1872, and the comparatively 

 frequent earth(|uakes in the Sierras and the Coast ranges make it 

 quite possible that these mountains are now being elevated as actively 

 as at any time in their history. In view of the great variety of the 

 country in which the stations were located, both as to topography 

 and geologic activity, in view of the great amount of material being 

 continually eroded from one region and deposited in another, thus 

 tending to overthrow the isostatic equilibrium, and in view of obser- 

 vations in other parts of the world, we are driven, with Dr. Hay- 

 ford, to the conclusion that isostasy is not an accidental condition 

 existing at the present time within this country, but is due to the 

 fact that the earth yields plastically to the long continued action of 

 even small forces. We feel justified, therefore, in believing that 

 isostatic equilibrium exists in other parts of the world and existed 

 in other geologic ages, and in saying that the whole earth is, and 

 always has been in isostatic equililjrium. 



This conclusion carries with it many important consequences and 

 has a very direct bearing on the theories of the origin of mountain 

 ranges ; for it tells us that every segment of the earth, having an 

 equal area of surface and with its a])ex at the center, contains the 

 same amount of material, which it is impossible either to increase 



