290 B. WILLIS DISCOIDAL STRUCTURE OF THE LITHOSPHERE 



others, in New England at least, occnrred during the middle Paleozoic; 

 still others invaded the entire belt from Newfoundland to Georgia in the 

 late Paleozoic; the latest eruptions are of Triassic age. They all show 

 the same general control of orientation. Even though many masses are 

 rounded, the distribution of such masses and the directions taken by the 

 offshoots from them coincide with the foliation of the gneiss and cor- 

 respond to the parallelism of the foliation, the lenticular intrusions, and 

 the outline of the Atlantic deep. As a phenomenon which has persisted 

 from pre-Cambrian time to the present, the parallelism of structure and 

 oceanic outline is an impressive fact. 



Along the Pacific coast of the Americas there is an enormous develop- 

 ment of batholiths associated with ancient and Mesozoic rocks and follow- 

 ing the continental border with marked persistence. The linear form 

 of the batholiths, regarded as intrusions, demonstrates the existence of 

 an earlier foliation parallel to their present trend. For North America 

 we know from the evidence of xilgonkian and Cambrian sediments and 

 their fossils that there was land in the Pacific border region during those 

 periods.*^ It became submerged beneath epicontinental seas, to a greater 

 or less extent at different periods and emerged as a whole in the late 

 Mesozoic, the uplift accompanying the intrusion of the great batholiths. 



Along the Pacific as along the Atlantic border, the controlling structure 

 was developed in the continental margin during an early geologic period, 

 when erosion had removed an upper layer down to the metamorphosed 

 and granitic rocks of the pre-Cambrian — that is, when the zone had been 

 unloaded and adjacent areas had been loaded. 



In general terms, then, though without the same persistence of land 

 conditions, the Pacific coast shows a relation between a deeply eroded sur- 

 face and the outcrop of steeply dipping schists, whose orientation is 

 what it should be if it were controlled by the elastic stress resulting from 

 unloading. 



If the bearing of the place relations of foliation to erosion and loading 

 be regarded as favorable to the recognition of a causal relation, it becomes 

 pertinent to push the inquiry to the further test of more detailed studies. 

 This has been done for the Coast ranges and Sierra Nevada of California, 

 and the deductions from the facts of local orogeny are entirely in harmony 

 with the inferences from general relations. 



The characteristic mountain form of the Pacific ranges is a tilted or 

 rotated block. If its under surface be conceived to be one of curved 



*8 C. D. Walcott : Cambrian. U. S. Geol. Survey. Correlation Papers. Bull. 81, 1891, 

 pi. ii (section). 



Ibid., Evolution of early Paleozoic faunas in relation to their environment. In out- 

 lines of geologic history, Willis and Salisbury, 1910, pp. 28-29. 



