AMERICAN COAL FIELDS 415 



West Virginia, eastern Kentucky and Tennessee, to northern Ala- 

 bama. In this field the measures are thinner than in Nova 

 Scotia ; the beds are thickest along the Appalachian shore line, 

 about 4000 feet in western Pennsylvania and 3000 in Alabama, 

 thinning much to the westward. 



(3) In Michigan the measures are only about 300 feet thick 

 and were doubtless laid down in an isolated basin. 



(4) The Indiana-Illinois field, which extends into Kentucky, is 

 from 600 to 1000 feet thick. 



(5) The Iowa-Missouri field extends southward around the 

 Palaeozoic island of southern Missouri into Arkansas and Texas. 

 In Arkansas the Carboniferous system, as a whole, attains a greater 

 thickness than anywhere else in North America. 



The two latter fields are separated by a very narrow interval, 

 and almost certainly were once continuous ; the Indiana-Illinois 

 field was probably also connected with the Appalachian area 

 across western Kentucky and Tennessee. 



As the coal measures are traced westward into Kansas, Nebraska, 

 and adjoining states, we find them dipping beneath strata of a very 

 much later date. When they once more return to the surface, as 

 in the Rocky Mountain region, they appear under an entirely new 

 aspect, being here altogether marine and containing no coal. In 

 Arizona and Utah a very large area is covered by Carboniferous 

 limestones and sandstones, and they form much of the thickness of 

 the lofty plateaus through which the Colorado has cut its canons. 

 Carboniferous beds occur around the Colorado island, in the Black 

 Hills, westward in the Uinta and Wasatch Mountains, and over 

 eastern Nevada. The thickness of the beds increases from the 

 Rocky Mountains westward, reaching 13,000 feet of limestones 

 and quartzites, for the whole Carboniferous system, in Utah and 

 Nevada, where deposition seems to have gone on uninterruptedly 

 from the Cambrian. In western Nevada was the Pacific shore, 

 beyond which Carboniferous beds reappear in California and Brit- 

 ish Columbia, extending over the interior plateau of the latter 

 region as far as 55 N. lat., and perhaps much farther. Many 

 Arctic islands have Carboniferous strata. 



