346 A. KEITH OUTLINES OF APPALACHIAX STRUCTURE 



under this theory would have been thrust northwestward by spread of 

 the sub-Atlantic crust. As a matter of fact, an important landmass lay 

 to the east of them during the late Paleozoic. On the Pacific border of 

 North America, while the mountain ranges border the ocean in accord- 

 ance with the theory, their thrust is toward the ocean. Another require- 

 ment of this theory is the permanence of the ocean basins, for if the 

 density was originally sufficient under the Atlantic, for instance, to have 

 caused the northwestward thrust of the Appalachians, such density would 

 be expected to continue and could not be shown to have discontinued. 



A further deficiency in this theory is a quantitative one. Even if the 

 utmost known differences in density of the crust are granted, a very 

 limited exjoansion landward in terms of miles is provided. This utterly 

 fails to meet the requirement of 200 miles of horizontal motion in the 

 Appalachians, for the distance from the center of the Atlantic to the 

 Appalachians is not much more than 1,000 miles. The density differ- 

 ences no doubt develop an enormous downward pressure, but the down- 

 ward motion (and also the lateral motion) is limited by the compressi- 

 bility of the rocks, which is of an order far dift'erent from 200 miles in 

 1,000. Finally, if this suboceanic spread is to become especially active at 

 the sea margin, the entire mass of the crust must be moved and its 

 strength and its friction overpowered. 



Furthermore, this difference of density under the Atlantic must l^e 

 assumed to have been pre-Paleozoic. Therefore, if deformation were 

 simply a matter of suboceanic weight of solid rock, why did it wait for 

 its action until the end of the Paleozoic era ; or, if it were satisfied in the 

 Precambrian deformations, what was left for the Paleozoic revolution? 

 In short, this theory as an explanation of the Appalachians fails to meet 

 the quantitative test of the known distances, times, and physical constants 

 unless another factor is introduced, as in the theory of batholiths. 



ISOSTASY 



Another theorv of the production of mountain rano-es has been strono-lv 

 held by many geologists in recent years. This theory, proposed by Bab- 

 bage and Herschel more than a century ago, Avas definitely put forward 

 in 1892 by Button, and was supported by many geologists of note. This 

 theory — that of isostasy — supposed that the lands rose because they were 

 made lighter by erosion and that the sea-bottom was depressed by the 

 growing load of sediments. In order that this process should be a con- 

 tinuing and potent cause. Button postulated some sort of transfer of the 

 underlying portions of the crust from beneath the oceans to a new place 



