ISOSTASY . 349 



very large piling up of sediment before the gravity differences would be 

 great enough to overcome the friction and the strength of the rocks and 

 thus to operate. The actual piles of strata known to us are from 20,000 

 to 30,000 feet thick at the most. These masses are found only in narrow 

 "belts 50 to 100 miles wide near the sea margins. Away from the maxima 

 the totals diminish rapidly and a thickness of one mile might be con- 

 sidered a fair average for the general area of deposition. It must be 

 admitted that this is a small agent for producing the enormous disloca- 

 tions and transfers of the crust in folded mountains. 



It has often been pointed out that the crust is capable of supporting 

 great loads, and it can not be denied that it is strong enough to support 

 large groups of high mountains above the general level. There are two 

 such areas in the Appalachians, each containing about 2,000 cubic miles 

 of matter above the adjoining valleys. The rocks concerned are Pre- 

 cambrian granites, diorites, and gneisses of very high specific gravity, 

 and they pass without change from the mountains down into the adjoin- 

 ino- lowland. It is obvidus that the earth's crust there sustains laro;e 

 overloads of heavy rock, yet the mountains have not only been carried 

 but have risen no less than 4,000 feet since the Mesozoic. This rise was 

 accomplished in five distinct stages, which indicates that the crust could 

 not only raise and support the load but that it could hold it through 

 five long periods of rest shown by as many successive peneplains. The 

 rigidity of the crust thus shown indicates clearly the order of resistance 

 that must be overcome by the weight of a mile-thick layer of sediment 

 before isostasy can act. 



Granted, for the sake of argument, that the initial gravities do differ 

 sufficiently to start the isostatic process, what, then, is to stop it? As 

 the deficiency of mass under the land was supplied by the underflow the 

 land would continue to rise, continue to be worn, sediments would con- 

 tinue to be deposited, and the sea-bottom would continue to descend. 

 This appears to be a process which is necessarily continuous if once 

 started, and it has the advantage of requiring only transfer of and no 

 final loss of matter, of heat, or of any force. 



This continuing process, however attractive it may seem, is easily sub- 

 jected to the test of observed geologic facts. Do the oceans continue to 

 deepen and the land continue to rise or not? The facts of geology em- 

 phatically say that they do not. Not only is this process not continuous 

 but it has been repeatedly reversed. Great areas of sea-bottom in the 

 Appalachians and elsewhere have been laid bare and their sedimnts worn 

 away, even after deposition had been long established. Again and again 

 during the Paleozoic this has occurred. A case of it followed the Silu- 



