METHOD OF ATTAIXIXG HORIZONTAL MOTION 371 



The attitude of the planes of layering and sheeting is of first impor- 

 tance in determining the direction of pressure put on the country rock 

 by the batholiths. The planes of parting as we see them exposed at the 

 surface have high dips — from 45 degrees to vertical. Over enormous 

 areas vertical dips are more common than any other. It is^ therefore, 

 clear that, when these highly inclined schists part, the openings also will 

 have high inclinations to the surface of the earth and that the intruding 

 masses will have similar attitudes. That such was actually the case can 

 be seen at countless localities in the southern Piedmont of the Appa- 

 lachians. The separation of these highly inclined layers is therefore 

 largely in the horizontal direction; in other words, in the direction 

 needed to accomplish the observed lateral thrust. 



It may be argued that part of this steep dip in the Precambrian is due 

 to folding during the Appalachian revolution, and such may be the case 

 in part. There were, however, strongly inclined Precambrian rocks be- 

 fore the Appalachian revolution, and the structures due to that revolu- 

 tion are imposed on those of earlier date. Furthermore, high dips of the 

 foliation and schistosity are characteristic of the Precambrian through- 

 out America, and in many areas where Paleozoic or later folding is 

 gentle, such as the domes of the Adirondacks, of Wisconsin, and of the 

 Black Hills, there is much high tilting of the Precambrian layers. It is 

 clear, therefore, that there were steeply inclined partings in the Pre- 

 cambrian rocks developed before even the deposition of the Paleozoic 

 recks, so that the structural conditions of steep parting planes were 

 present, regardless of what ha23pened later in the Appalachian folding. 



We can not be sure, however, that this general high inclination of the 

 rock partings goes down indefinitely into the crust. Indeed, it is prob- 

 able that such is not the case. With increasing deiDth below the surface 

 the rock pressures are greater in the vertical direction, which would 

 naturally result in the production of partings more and more nearly 

 normal to that pressure; in other words, approaching the horizontal. 

 This condition must yield to another still farther downward in which 

 the strength of the rocks is Avholly OA^ercome by the vertical pressure^ 

 perhaps to such an extent that they have a potential freedom to move in 

 any direction. This might produce a condition resembling fluidity, but 

 one which would require the application of enormous force to cause any 

 change in the position of a particular mass. 



There is some evidence that this condition of horizontal partings does 

 exist in the deeper masses. Of course, approximately horizontal partings 

 are by no means unknown in the Precambrian rocks, but the difficulty 

 lies in ascertaining how deeply they were buried. The determination of 



