EVOLUTIOX OF MODERN SURFACE FEATURES 9 



appraised critically on their individual merits. Thus, the supposed 

 borderland of "Cascadia" which has been postulated along the 

 Pacific Coast of North America, if it existed, could hardly have 

 extended beyond the edge of the present continental shelf, the sub- 

 merged part of the continental crust. 



This is not to say that certain modifications of the doctrine of 

 permanence are unworthy of consideration : 



Some geologists believe that the plates of continental crust, al- 

 though permanent, have drifted through time across the subcrust, so 

 that their positions have shifted with respect to other continents, 

 and to the poles. Although there is a great deal of persuasive evi- 

 dence for such an interpretation, much more evidence, both geologi- 

 cal and geophysical, is against it. The possibility of continental drift 

 need not concern us greatly in our present problem ; even under such 

 an hypothesis North America has long retained about the same 

 position with respect to Asia, South America, and the Pacific Ocean. 



Other geologists believe that, although present oceanic areas are 

 unlikely ever to have been continental, the continental plates may 

 have increased in area through time by processes of accretion — by 

 building of sediments over the edges of the oceanic crust, and their 

 subsequent consolidation into continental crust during mountain 

 building. Western North America may have increased in area, 

 rather than diminished through recorded geologic time, by incre- 

 ments along its Pacific margin, especially during the orogenic period 

 of the latter half of Mesozoic time. The area of the Coast Ranges of 

 California, for example, may have been continental during only the 

 last 100 million years of geologic time; before that, open ocean. 



GEOSYNCLINAL PHASE 



General Concepts 



Growth of a mountain system ordinarily begins with a geosynclinal 

 phase, or time of quiet preparation, when marine sedimentation 

 went on over the site of the future mountain belt. A geosyncline is an 

 area where sedimentation proceeded actively, to the accompaniment 

 of more or less crustal movement (Kay, 1947). Many geologists have 

 believed that the geosynclines of North America were features that 

 developed within the continental platform, between a central nucleus 

 and the "borderlands" along the edge. Now, there is a growing 



