480 THE CRETACEOUS PERIOD 



were again submerged, and a wide sea connected the Gulf of 

 Mexico with the Arctic Ocean. The eastern coast of this interior 

 sea began in northwestern Texas, running through Kansas and 

 Iowa nearly to the present line of the Mississippi River. West- 

 ward the coast-line was the uplift which ran from southern Mexico 

 into British Columbia. The Colorado region was again converted 

 into islands. North of the Great Basin land the interior sea was 

 connected with the Pacific and Arctic Oceans, which united over 

 the northwestern part of the continent. 



On the Pacific side, the Sierras, which had suffered greatly from 

 denudation, were again folded, and separated from the interior 

 basin by a fault, while a fracturing of the crust began the system of 

 Basin Ranges, arching upward the surface of the Great Basin. A 

 moderate transgression of the sea caused the Upper Cretaceous to 

 extend farther east than the Lower. Volcanic activity continued 

 and immense bathyliths were formed deep within the mountains. 

 The sea extended from Lower California northward along the Sierra, 

 into eastern Oregon at the foot of the Blue Mountains. 



The North American continent was thus divided into two prin- 

 cipal land masses, the larger one to the east and comprising the 

 pre-Cambrian and Palaeozoic areas. In the limits of the United 

 States this land lay almost entirely east of the Mississippi, except 

 for a southwestern peninsula, including Missouri, Arkansas, Okla- 

 homa, and part of Texas. The western area was much smaller, 

 extending from southern Mexico into British Columbia, and hav- 

 ing its greatest width between the fortieth and forty-fifth parallels 

 of latitude. Between the two lands lay the Colorado islands, and 

 doubtless many smaller ones as well. 



The character of sedimentation differed so much in the various 

 regions of the continent that the subdivisions of the Upper Creta- 

 ceous have received different names in the separate provinces, and 

 only approximately correspond in time. 



Along the Atlantic border the Upper Cretaceous strata are a 

 series of marine sands and clays, which are still almost horizontal 

 in position and of loose, incoherent texture. In New Jersey there 

 are extensive developments of green sands (see p. 175), locally 



