124 GEOLOGY 



unconformably on the Triassic system, and contains some volcaiii' 

 material and, locally, some coal. 



Farther north, the Lower Cretaceous has not always been separated 

 from the Upper, but the former has extensive development in some 

 parts of northern Alaska, 1 where it locally contains coal, and is known 

 even north of the Arctic circle. It is also believed to occur on the west 

 coast of Greenland, opposite Disco island. From the fossils, the Green- 

 land beds are believed to represent some such horizon as that of the 

 Kootenay, or Potomac. 2 



Panama. — Conglomerate of Early Cretaceous age is said to occur 

 on the isthmus of Panama, 3 its materials having been derived from 

 the south. The Cretaceous beds here rest unconformably on forma- 

 tions of late Jurassic (probably) age. 



The Close of the Comanchean (Lower Cretaceous) Period. 



In the latter part of the Comanchean period, or at its close, there 

 were considerable changes in the geography of the continent. Along 

 the Atlantic and Gulf borders were changes (perhaps before the close 

 of the period) which converted considerable tracts of the known Potomac 

 and Tuscaloosa series from areas of deposition to areas of erosion. 

 In Texas, the sea was withdrawn, and the Comanchean system was some- 

 what deformed and faulted, while in Mexico the deformation of the 

 sj^stem was notable. Following these changes, the Comanchean sys- 

 tem was subjected to prolonged erosion. Geographic changes also 

 affected the western coast. Locally, as in the southern Coast range 

 of California, there was folding of the Lower Cretaceous beds, 4 and 

 volcanic activity, while in other places the sea spread itself over areas 

 which had been land. Still other areas appear to have emerged at 

 this time, and never to have been again submerged. 5 



On the whole, therefore, the deformative movements at the close of 

 the Early Cretaceous period were considerable. They were more 



1 Schrader, Bull. Geol. Soc. Am., Vol. XIII, pp. 245-6, Professional Paper, 20, 

 pp. 72-77; Mendenhall and Schrader, Professional Paper, 15, p. 37; and Collier, 

 Bull. 218, U. S. Geol. Surv., pp. 15-17. 



2 White and Schu chert, Cretaceous Series of the West Coast of Greenland, Bull 

 Geol. Soc. Am., Vol. IX, pp. 343-368, 1898. 



3 Hershey, Bull. Dept. Geol. Univ. of California, Vol. 2, pp. 240-249. 



4 Fairbanks, Jour. Geol., Vol. Ill, pp. 415-430, and San Luis folio, U. S. Geol. Surv 



5 Kansome, Bisbee, Ariz, folio, U. S. Geol. Surv. 



