826 HISTORICAL GEOLOGY. 



Comox, and in the Bow River region, north of Montana, probably in beds of 

 Montana age (Dawson). 



But the coal-beds are mostly in the Laramie formation. They are worked 

 for coal in Colorado, Utah, Wyoming, Montana, and Kew Mexico. In Colo- 

 rado alone the coal-fields have an aggregate area of about 18,000 square miles 

 (R. C. Hills, 1892). The beds are often five to six feet in thickness, and 

 one at Evanston, in western Wyoming, has been described as 26 feet thick. 

 In British America, at Edmonton (113^-° W. 53^° N.), and in the Souris dis- 

 trict, there are Laramie coal-beds. 



In Gunnison County, Col., at Crested Butte, a bed of anthracite five feet 

 thick is worked ; and in New Mexico, at the Old Placer Mountain, eight miles 

 east of San Antonio, is another locality of anthracite. The anthracite is a 

 result of alteration by the heat of eruptive rocks. 



To appreciate the position and width of the Cretaceous seas over the 

 western Continental Interior during the Colorado and Montana epochs, and 

 especially the Niobrara portion of the former, the reader should refer again 

 to the map on page 813 ; and, still better, to some colored geological map of 

 North America. 



Their eastern border extended, from what is now western Texas, east- 

 ward and northward over central Kansas, and thence along eastern 

 Nebraska and Dakota into British America. In the western portion of 

 these interior waters there were the large Archaean islands of the protaxis, 

 high lands and low lands varying in limits with oscillations in level, which 

 were mostly forest-clad, and well populated, as evidence shows, by Mammals, 

 Amphibians, and Reptiles, the Reptiles taking the lead in size and power. 

 Beyond these islands the seas spread still westward over nearly all of 

 Wyoming and Utah to a line passing southward through Great Salt Lake, 

 where the western shores lay along the lands of the Great Basin. 



In the progress of the Upper Cretaceous, the non-marine Dakota epoch 

 was followed by a second, the Colorado, in which the Interior sea gradually 

 attained ocean-like conditions, and was inhabited by great Mosasaurids or 

 Pythonomorphs, and Sea-Saurians related to the Plesiosaurs, as well as 

 Sharks and Saurodont Pishes. Even before the Niobrara beds had all been 

 deposited, a shallowing had begun in Kansas. S. W. Williston states that 

 in the beds of Kansas Invertebrates abound; that Reptilian remains are 

 unknown in the lower part of the Niobrara beds within 100 feet of the base, 

 but higher up are common fossils. " Species of two or three genera of 

 Mosasaurs occur at different levels, but those of Clidastes [Edestosaurus 

 of Marsh] only in the upper part. Turtles are rare in the lower portion, 

 while very common in the uppermost beds." 



This shallowing was general over the Continental Interior as the Colorado 

 epoch closed. Moreover, the Colorado fauna, in some unexplained way, 

 disappeared. During the Montana epoch the waters, however, were still 

 salt, and marine life was abundant, and included Plesiosaurids. But the 

 shallowing was continued ; and in the following Laramie epoch the waters 



