ULTIMATE FORCES 377 



diverted into the horizontal in order to explain the observed results. In 

 the previous section a mechanism for attaining this diversion was out- 

 lined, the principal requirement of which was hydrostatic pressure. If 

 that condition is granted, and it is difficult to deny that it exists, the 

 means of diverting the downward pull of gravity seems ample. It will 

 no doubt be conceded by advocates of isostasy at least that such fluidity 

 is reasonable and proper at some depth, in view of the mobility which 

 that theory requires even from solid rocks near the surface, not to men- 

 tion the return flow of deep-seated matter from beneath the sea to the 

 land. 



For final analysis of the problem, let it be granted that an excess 

 pressure of gravity was transformed into lateral pressure through batho- 

 liths whose magmas were practically in a hydrostatic condition and that 

 this pressure built the mountains. The questions next to be answered 

 are these, On what part of the crust was this excess pressure exerted and 

 why was it exerted? 



The direction in which lay the unduly pressed crust can be readily 

 inferred from the direction in which the crust moved when yielding. 

 That motion was from the southeast to the northwest, in the case of the 

 Appalachians, and it is on the southeast, therefore, that the heavy part 

 of the crust was situated. Since the effects extend from the Gulf of 

 Mexico to ISTewfoundland and probably across the North Atlantic into 

 Europe, the locus of greatest pressure M^as probably under the Atlantic, 

 where the normals to the trends of the system intersect. 



What, then, are the features of the Atlantic that bear on the second 

 question? Two major points are crucial: First, it has been fairly well 

 determined by the observations of years that the rocks visible on the 

 shores and islands of the oceans are denser as a whole than those of the 

 continents. Second, gravity observations made instrumentally in recent 

 years seem to show that the force of gravity, and therefore the densities, 

 is greater in and around the oceans than on the continents. Mathe- 

 matical analyses of gravity differences are made which indicate that they 

 are compensated within and do not extend below a depth of 60 miles. 



If, then, we accept as a fact the greater density of the crust under the 

 oceans, there is provided a downward pull of gravity in one region 

 greater than in adjoining regions. This would tend to depress the ocean 

 bottom. A depression could be actually accomplished by further con- 

 densing the underlying rocks or by lateral movements of the crust. It 

 is reasonably sure that all condensation compatible with other conditions 

 took place at an early stage, so that depressions to satisfy existing densi- 

 ties would be attained by lateral movements. If by any means any down- 



