300 



HISTORICAL GEOLOGY. 



to ten feet. The number of seams in a single field may 

 be a hundred or more, and their aggregate thickness may 



FIG. 230. Panther Creek and Summit Hill traverse. (After Daddow.) 



PIG. 221. Illinois coal-field. (After Daddow.) 



be, in some cases, one hundred to one hundred and fifty 

 feet of solid coal. 



Coal-Fields of the United States. In the map on 



page 272 the coal-fields of the United States belonging 

 to this period are represented in black. It is seen that 

 there are four of these : 1. The Appalachian coal-field, 

 probably the richest in the world. In a general way it 

 may be said to cover the western slope of the Appalachian 

 chain from Pennsylvania southward. It covers an area 

 of 60,000 square miles. 2. The central coal-field. This 

 covers nearly the whole of Illinois, the western portion of 

 Indiana, and northwestern Kentucky, and its area is 47,000 

 square miles. 3. The great Western coal-field. This cov- 

 ers southern Iowa, northwestern Missouri, eastern Kan- 

 sas, the Indian Territory, western Arkansas, and north- 

 ern Texas. Its area is no less than 78,000 square miles. 

 4. The Michigan coal-field. This occupies an area of 



