186 C. SCHUCHERT THE XORTH AMERICAX GEOSYNCLINES 



Ozarkian, and early Ordovician deposits are ver}^ thick througlioiit the 

 geosyncline, varying between 6,000 and 16,000 feet. It is the grandest 

 known Cambrian sequence anywhere in the world, and our knowledge 

 of it is almost wholly due to Walcott. The Middle and Upper Ordovi- 

 cian and all of the Silurian may be absent or are poorly represented, 

 apparently by not over 500 feet in thickness, in the medial length of the 

 geosyncline, while the later Devonian is usually present, though never 

 thicker than 1,000 feet. About middle Mississippian time a gTeat 

 change took place in the extreme northern portion of the trough, since 

 the whole Mackenzie region was then warped above sealevel, and this 

 area did not again have deposits until Jurassic time. Therefore it is 

 only in the southern two-thirds of the Cordilleran geosyncline that the 

 later Mississippian, Penns^dvanian, early Permian, and Triassic are 

 present in thick deposits, varying in depth between 1,500 and 5,500 

 feet. In other words, we agree with Ulrich that the Paleozoic sequence 

 in the Cordilleran seas is far less complete than that of the Appalachian 

 geosyncline, and that the latter area is best "fitted to fill the require- 

 ments of a standard'^ for stratal correlations in America. These con- 

 clusions therefore appear all the more strange when we take into con- 

 sideration the fact that the Cordilleran seas, wdien present, were often 

 more extensive than were those of the Appalachian trough. 



RISE OF THE AXCESTRAL ROCKY MOUXTAIXS GEAXTICLIXE 

 (See Maps. Figiires 2 and 12) 



^\e must now make a little digression to explain the rising of a 

 geanticline in the eastern portion of the Cordilleran geosyncline, one 

 that is clearly described by Lee (see page 286) as the Ancestral Southern 

 Rocky Mountains. All through the earlier Paleozoic the Cordilleran 

 seas transgTessed eastward on and at times across the neutral region 

 Siouia. In the same way the Mississippian seas spread westward variably 

 over parts of Siouia. These conditions continued until late in the 

 Tennesseean. Then, apparently early in Pennsylvanian time, at least 

 all of eastern Colorado and Xew Mexico, western Kansas, western 

 Oklahoma, and northwestern Texas was bowed up into the high Ancestral 

 Rocky Mountains geanticline. To the west of this highland lay the 

 Cordilleran seas of late Paleozoic time, while to the east of it the Mis- 

 sissippian seas of late Pennsylvanian time and the following Red Beds 

 Permian overlapped to the westward. In other words, the source of 

 sediments for these seas was completely changed by this geanticline. 

 Finally, we learn from Lee that the whole of this arch was eroded to 

 sealevel by late Jurassic time, since the Logan sea, and more especially 

 that of the Cretaceous, completely transgressed it. 



