CARBONIFEROUS AGE. 293 



Subcarboniferous rocks of the age ; but it is probable that formerly 

 the coal fields stretched across the channel of the Mississippi, and that 

 the present separation is due to erosion along the valley. 



Besides these, there are the following barren of coal, or nearly so. 



IV. The Rocky Mountain and Pacific Border Regions. — 1. The 

 great Basin and Summit area, embracing parts of Montana, Wyoming, 

 Colorado, Utah, and Nevada. 



2. The California area, in the northern half of California. 



V. The Arctic Region. — In Melville Island, and other islands be- 

 tween Grinnell Land and Banks Land, on Spitsbergen^ and on Bear 

 Island north of Siberia. 



The extent of the coal-bearing area of these Carboniferous regions is approximately 

 as follows : — 



Rhode Island area 500 square miles. 



Alleghany area .....*.. 59,000 square miles. 



Michigan area 6,700 square miles. 



Illinois, Indiana, West Kentucky .... 47,000 square miles. 



Missouri, Iowa, Kansas, Arkansas, Texas . . . 78,000 square miles. 



Nova Scotia and New Brunswick .... 18,000 square miles. 



The whole area in the United States is over 190,000 square miles, 

 and in North America about 208,000. Of the 190,000 square miles, 

 perhaps 120,000 have workable beds of coal. 



1. SUBCARBONIFEROUS PERIOD (13). 

 I. Rocks : kinds and subdivisions. 



In the Interior Continental region, the Subcarboniferous rocks are 

 mainly limestones. They are largely displayed in Illinois, Kentucky, 

 Iowa, and Missouri, and have at some points a thickness of 1,200 feet. 

 They also occur in Arkansas and Texas. To the eastward, the pro- 

 portion of limestone diminishes. In Tennessee, the lower beds are 

 siliceous, and the upper, limestone. In Michigan, there are about 

 seventy feet of limestone, resting upon 480 feet of shales and sand- 

 stones ; in Ohio there are over 600 feet of sandstones and shales, with 

 twenty feet or less of limestone at top, in some parts. 



In the Appalachian Region in Pennsylvania, the beds, instead of 

 being limestones, are sandstones or shales, excepting small portions in 

 the southwestern part of the State. The thickness increases from the 

 westward and northward toward Pottsville and the Lehigh region, 

 where in some places it is 4.000 to 5,000 feet, In Virginia, the beds 

 are more calcareous, and the limestone increases in amount to the 

 southwest, and continues to Alabama and Mississippi. 



There are thin workable seams of coal in some of these Subcar- 

 boniferous beds of Pennsylvania, Virginia, western Kentucky, and 

 southern Indiana, and also valuable beds of clay-ironstone. 



