PALEOZOIC ROCKS AND ERA. 



301 



6,700 square miles in the center of the Michigan Peninsula. 



Besides these, there is a small area of coal of little value 



in Rhode Island, and a fine 



coal-field of 18,000 square 



miles accessible to us in 



Nova Scotia. Of the 



102,200 square miles of 



coal within the limits of 



the United States, 120,000 



square miles are estimated 



as workable. It may be 



said with confidence that 



there is no country in the 



world so liberally supplied with this great agent of modern 



civilization as our own. 



Origin of Coal and of its Varieties. 



There can be no doubt that coal is of vegetable origin. 

 All portions of a coal-seam, even the most structureless 

 to the naked eye, when properly prepared, reveal their 

 vegetable structure to the microscope (Figs. 222, 223). 



Appalachian.. . 



Central 



Great Western. 

 Michigan . . 

 Rhode Island. . 



Nova Scotia. 

 Total. . 



60,000 



47, GOO 



78,000 



0,700 



500 



192,200 

 18,000 



210,200 



FIG. 222. 



Section of anthracite : a, natural size ; b and c, 

 magnified. (After Bailey.) 



FIG. 223. Vegetable 

 structure in coal. 

 (After Dawson.) 



Varieties of Coal. Assuming the vegetable origin of 

 coal, how do we account for the varieties ? These varie- 

 ties are of three kinds : 1. Varieties depending on degrees 



