164 C. SCHUCHERT THE NORTH AMERICAN GEOSYNCLINES 



strike of which is northwest-southeast^ in harmony with the present 

 trends of the Pacific system of mountains in California-Oregon, and 

 again in Vancouver-British Columbia. In any event, there were, since 

 the Tennesseean, northwesterly trending channels across Cascadia, the 

 Sliastan channel in the south and the Alexandrian emhayment in the 

 north, dividing Cascadia into a smaller southern mass that may be known 

 as the Calif ornian borderland, and a much longer northern one that may 

 be called the Charlotte borderland (see figures 3, 9, and 13). 



There is as yet no evidence to show that Cascadia as a whole underwent 

 marked orogeny at any time until near the close of the Jurassic, although 

 in the late Devonian there was orogeny going on, together with volcanic 

 action, in northern California. Diller has shown that here the Tennes- 

 seean lies with an angular unconformity on the Devonian. On the basis 

 of the sediments of the Cordilleran geosyncline, however, it is clear that 

 Cascadia must have been a highland during the Proterozoic and again in 

 early Cambrian time. Toward the close of the Cambrian it was a low- 

 land and remained so until early Carboniferous time, when it was reele- 

 vated to furnish the moderately thick formations of the Tennesseean and 

 Pennsylvanian formations. 



Medial or nuclear Area of N'orth America 



(>See Map. Figure 3) 

 GEXERAL DI8CU88I0X 



Along the inner sides of the borderlands lie the several comparatively 

 narrow geosynclines, Avhose waters extended at times irregularly over the 

 medial area of the continent, the very extensive and ancient nucleus of 

 North America. The structure of this medial area came into being long 

 ~l)efore the Cambrian, and in the main during the earlier Proterozoic, 

 though mountains arose here as late as late Proterozoic time (Killarney 

 Mountains). ^^ It was therefore the oldest part of the continent, made 

 stiff and rigid through a vast amount of orogeny. Ever since the Pro- 

 terozoic the nucleus of the continent has lain but little above sealevel, 

 warping periodically up or down some hundreds of feet. Repeatedly, 

 shallow seas have formed over parts of it, and yet in no place have the 

 Paleozoic sediments accumulated to a depth of one mile. Usually these 

 -strata are measured in hundreds of feet rather than in thousands. Be- 

 cause the nuclear part of North America has remained so near sealevel 



1^ W. H. Collins : An outline of the phjsiographic history of northeastern Ontario. 

 ..Tour. Geology, vol. 30. 1922. pp. 206-207. 



