208 C. SCHUCHERT THE NORTH AMERICAN GEOSYNCLINES 



dilleran one there developed in Mesozoic time two sequent geosynclines- 

 (see map, figure IG). The amount of sediment in any of these troughs 

 appears to have nothing to do with the time when they were folded into 

 synclinoria. In places there may be even less than 10,000 feet of sedi- 

 ments, on the average the amount of subsidence is between 15,000 and 

 35,000 feet, while in the mid-length and width of the combined Cordil- 

 leran-Eocky Mountain geosynclines there was about 76,000 feet, or about 

 1-1 miles depth, of strata before the troughs evolved into synclinoria! 

 mountains. Xor has the length of geologic time anything to do with the 

 vanishing of the geosynclines. Both the Saint Lawrence and the Acadian 

 troughs endured only from early Cambrian time to the close of the De- 

 vonian ; the southern Appalachian one was in formation from the early 

 Cambrian into the Pennsylvanian ; while vast stretches of the Cordilleran 

 geosyncline did not fold from early Proterozoic time until the close of 

 the Cretaceous, even though other parts of the trough rose into a verj 

 long and high geanticline during the middle Mesozoic. In all of this, we 

 appear to see that the balancing of masses under the law of isostasy con- 

 tinues until some other force changes the relations of things deep down 

 within the lithosphere. 



So far as North America is concerned, we see that no primary geosyn- 

 clines have come into existence after the Cambrian, and that the larger- 

 ones were clearly established either late or early in the Proterozoic. Se- 

 quent geosynclines, of as great importance as any (Rocky Mountain), 

 along wdth vast geanticlines (Cordilleran Intermontane), originated in 

 Mesozoic time, but their evolution was out of a very wide and primary 

 geosyncline (Cordilleran). We therefore conclude that in ISTorth America, 

 no new deeply subsiding areas have originated since the early Proterozoic. 



It may be that the present highly elevated condition of the Cordilleran 

 Intermontane geanticline will subside some thousands of feet. Should 

 this happen, it appears probable that an inland sea will form, extending- 

 the Gulf of Lower California into the Great Basin country. It will be, 

 however, a rift sea like the present Red Sea, formed through the in- 

 breaking of the Cordilleran Intermontane geanticline, and not a new 

 geosyncline. 



The narrower and shorter-lived geosynclines (= monogeosynclines)^ 

 seemingly conditioned by the smaller Atlantic and Arctic oceans, are in 

 striking contrast to the far wider, longer-enduring, and immensely more 

 diastrophically active geosynclines (==:polygeosynclines) conditioned by 

 the Pacific. We therefore ask, Has the vastness of the negative areas, the 

 oceans, any direct and sympathetic action upon the sides of the positive 

 continents? It would seem so. If this surmise is admitted, we must 



