DEVELOPMENT OF THE CORDILLERAN GEOSYNCLINE 185 



Cordilleran Intermontane geanticline. In Cretaceous times, then, there 

 lay on the western side of this geanticline the Pacific geosyncline, with a 

 length of at least 3,500 miles, and on its eastern one the Roclcy Mountain 

 geosyncline, or Coloradoan sea, extending in the form of a sigTHoid curve 

 from Behring Straits into the Caribbean mediterranean, a distance of 

 over 5,000 miles. 



EARLY CORDILLERAN GEOSYNCLINE 

 (See Maps, Figures 4 to 12) 



Just when the Cordilleran geosyncline came into existence is not 

 known, but it is certain that its central part was present early in 

 Proterozoic time, and seemingly with about the same position and extent 

 as in the early Paleozoic (see map, figure 1). Its presence is clearly 

 evidenced by the vast deposits of Proterozoic time in the Beltian series, 

 extending certainly from Great Salt Lake into British Columbia to 

 about 55 degrees north latitude. Furthermore, in the Grand Canyon 

 area of Arizona other thick Proterozoic deposits are known, so that we 

 may say that the Cordilleran geosyncline in Proterozoic time had a 

 regional distribution similar to that which it had during the early Paleo- 

 zoic. These Proterozoic deposits, mainly sandstones and shales and with 

 very little of limestones or igneous materials, var}^ in thickness from 

 15,000 to over 30,000 feet, and as their depth and coarseness increase 

 westward, it is also clear that even then Cascadia was, as later on, the 

 bounding western borderland. Probably the most extraordinary fact 

 in our present studies is that of the conformable relations of the Pro- 

 terozoic and Paleozoic strata. In other words, there was no marked 

 orogeny in the Cordilleran geosyncline at the close of the Proterozoic 

 as there clearly was in all the other marginal areas of Xorth America. 

 Can this tranquillity of the American Cordilleran region mean that the 

 North Pacific Ocean was not a decidedly sinking region during the 

 Proterozoic, and that this vast basin did not begin to become crustally 

 unstable until late Jurassic time, when the evidence of vast mountain- 

 making appears for the first time in many lands that bound the Pacific 

 Ocean ? As yet we know of only local orogeny in the Cordilleran region 

 of Xorth America during Proterozoic and Paleozoic time. Therefore 

 one of the extraordinary phenomena in the geologic history of the Cor- 

 dilleran region is the conformability of Proterozoic, Paleozoic, and even 

 Mesozoic formations up to the close of the Jurassic. 



The thicknesses of the Paleozoic formations throughout the western 

 part of the Cordilleran geosyncline vary between 10,000 and 23,000 feet, 

 being apparently greatest in the southern half. The Cambrian, 



