DEVELOPMENT OF THE DISCOIDAL HYPOTHESIS 289 



may serve as an illustration, the Atlas, xVpennines, Alps, and Carpathians, 

 around the depressions of the western Mediterranean, the Adriatic and 

 Po, and the Hungarian plain. The chain extends eastward across Asi^ 

 into the Mak}^ Peninsula and the curve of Sumatra and Java. Its course 

 lies around, between, and in the margins of basins, some of which ha^e 

 been obliterated in the Tertiary deformations, while others still persist. 

 Some are small and intercontinental, others are oceanic. The local struc- 

 tural phenomena are among the most complex known to geology, and it is 

 not the purpose of this article to discuss the facts or the mechanics of 

 their development, but they are more readily brought into accord with 

 mechanical principles if we recognize that the winding directrices of the 

 ranges are the outcrops of foliation, which is curved also in any vertical 

 section. 



The directrices traced by Suess are based on the strikes of the axes of 

 folds, on the strike of schistosity, and on the trends of intrusive rock- 

 masses. The three are intimately related, and schistosity is the funda- 

 mental one, for schistosity in deep-seated rocks guides the folding of 

 sediments and the course of intrusives. From this relation results the 

 parallelism which is so generally observed and which may be utilized in 

 the present discussion to trace the effects of the relation between foliation 

 and the postulated elastic stress. 



As a case in point, let us take up the zone of metamorphic rocks of the 

 eastern Appalachians from jSTewfoundland to Georgia. It is 3,000 kilo- 

 meters (2,000 miles) long and from 50 to 300 kilometers wide. It is a 

 continental feature. As an element of the continent it has persistently 

 stood relatively high and is one of the original nuclei, which has from 

 the earliest known geologic periods to the present formed a land between 

 the Atlantic basin and the epicontinental seas or lowlands of the interior. 

 It is therefore a mass which has been deeply eroded and in which, if 

 erosion directs foliation in the underbody, the dip of the exposed schists 

 should be steep and the strike of the schists should conform to the limits 

 of the eroded area, which, on the east, are the limits of the Atlantic basin. 

 Furthermore, intruded igneous bodies should have a similar orientation. 



The required relations of dip and strike exist throughout the belt.^^ 



The original foliation — at least, the most ancient foliation — character- 

 izes the Carolina gneiss, which is the matrix into which younger igneous 

 rocks have been intruded. The earlier intrusions are pre-Cambrian ; 



**See Geologic Map of North America. U. S. Geol. Survey, Professional Paper 71, 

 1910, in pocket ; also published separately ; and folios of the Geologic Atlas of the 

 United States, No. 192, Eastport, Maine ; No. 149, Penobscot Bay, Maine ; No. 83, New 

 Yorl£ City ; No. 162, Philadelphia ; No. 70, Washington, D. C. ; and No. 151, Roan Moun- 

 tain. 



