190 C. SCHUCHERT THE NORTH AMERICAN GEOSYNCLIXES 



Paleozoic time, thoiigli we may for the present assume this condition 

 because of the rehitively thin deposits of the Sonoran embayment. 



The Mexican geosyncline made its appearance in the north U\te in 

 Pennsylvanian time, then in the south late in the Tria^sic. more definitely 

 early in the Jurassic, and before the close of this period its waters had 

 attained Texas. Then there was a retreat of the sea from the north, 

 but early in the Lower Cretaceous a general transgression of the Mexican 

 sea set in, that widened until it finally covered more than two-thirds of 

 Mexico. It continued to spread northward into the United States, and 

 early in the Upper Cretaceous (Benton) the Mexican waters united witli 

 the Dakotan sea, tliat since late Lower Cretaceous time (Blairmore) had 

 l)een spreading southward. The maximum thickness of the Mesozoic 

 deposits in Mexico may be as great as 20,000 feet. 



The Arctic coast of Alaska is too little known to determine whether 

 there is here a geosyncline having a borderland facing the Arctic Ocean. 

 The older Paleozoic formations have been found to be some thousands 

 of feet thick, followed in the Cape Lisburne area by more than 5,000 

 feet of Lower Carboniferous. The Jurassic, wholly of continental strata. 

 is 15,000 feet thick, followed by about 5,000 feet of Cretaceous. This 

 resfion, for easier reference, mav be known as Endicoff sea. 



The stratal history of the Mackenzie Valley in its broader aspect has 

 been made known during the past ten years, and it is now established 

 that, beginning with the Lower Cambrian, there was no orogeny in this 

 reofion until late Cretaceous time. Xot even the late Jurassic risina' 

 of the Cordilleran Intermontane geanticline alfected the area of the 

 Mackenzie Valley or, for that matter, any of the region of the Kocky 

 Mountain geosyncline. Here during Cambrian to late Devonian times 

 the Cordilleran trough received no less than 13,000 feet of marine strata. 

 Then the region was warped above sealevel, and there are no strata of 

 any kind until late Jurassic time, when the overlaps of the British 

 Columbia geosyncline toward the east attained the region of the ^lacken- 

 zie Valley. The making of the Cordilleran Intermontane geanticline fol- 

 lowed, but in this region the arch apparently was not a highland, since 

 to the east of it the Cretaceous deposits do not exceed a few thousand 

 feet in thickness. 



It was during the late Lower Cretaceous (Blairmore-Albian) that the 

 Arctic Ocean began to spread south through the Eocky Mountain geosyn- 

 cline, but these marine waters seemingly did not extend south of the 

 Peace River of Alberta, though fresh-water coal-bearing strata of about 

 this time (Kootenai) occur southward through Alberta into Montana. 

 Then the seas retreated northward, or, better, their spreading appears 



