398 B. WILLIS A THEORY OF CONTINENTAL STRUCTURE 



Appalachia, the eastern element, which was a conspicuous land area 

 during the Paleozoic and is characterized by a sum of unconformities 

 nearly, if not quite, equal to that of Laurentia. Its eastern portion, 

 which was presiimably more extensive than the area now bounded by the 

 Atlantic, has been more or less submerged since the early Mesozoic. 



Llano and Ozarkia, the more conspicuous points in a zone where the 

 balance of unconformities and sediments is nearly even and their alge- 

 braic sum may be roughly estimated as zero, Avith perhaps a plus correc- 

 tion. 



The Eocky Mountain element, comprising Wyoming, Colorado, and 

 Arizona, which is distinguishable by a sum of unconformities that gives 

 it a positive character in spite of frequent submergence and is more 

 clearly recognized as a recurrent landmass when compared with the adja- 

 cent negative region of the Great basin. 



A Pacific element, occupying Xevada and a zone extending thence 

 northward to latitude 57° in British Columbia and westward to an un- 

 known distance, determined chiefly on the pre-Cambrian and Paleozoic 

 sediments derived from it. 



The Yukon element, an element comprisiiig the ancient rocks of the 

 Yukon plateau, which apparently has been the source of Paleozoic and 

 Mesozoic sediments of the Pacific and Arctic littorals. 



Possibly, but not probably, an isthmian element or elements repre- 

 sented by the ancient schists of Mexico and Guatemala. 



Negative Elements of North America 



By contrast with the positive elements of the continent wbich are recog- 

 nized by absence of sediments and preponderance of unconformities, the 

 negative elements are distinguished by the sediments which bury them. 

 The significant strata are practically limited to the Paleozoic and Meso- 

 zoic, as pre-Paleozoic terranes, with the exception of the Huronian, the 

 Belt, and the Ocoee, are too limited in occurrence to give tangible evi- 

 dence, and as the possible effects of post-Mesozoic isostatic adjustment 

 are profoundly modified by the compression and intrusion that mark the 

 Cordilleran orogenic activity. 



Through his studies of the Cambrian, Walcott* first distinguished the 

 eastern and western troughs which are the loci of the principal negative 

 elements, the Appalachian trough and that of Nevada and British 

 Columbia. 



* C. D. Walcott : The North American continent during Cambrian time. Twelfth Ann. 

 Kept. U, S. Geol. Survey, pt. 1, pp. 529-561, 



