COAL MEASURES. 75 



are subjected. The aggregate thickness of the coal-beds in the Pottsville district 

 is 120 feet, in the Wilkesbarre district 62 feet, and in the Pittsburg district 25J 

 feet. The thickest vein at Wilkesbarre is 29 feet, and at Pittsburg 8 feet. 

 The best seam in Ohio is from 6 to 12 feet in thickness, and is called the Hocking 

 River Coal-bed. 



160. The second coal-field in importance covers nearly two-thirds of Illinois, 

 the western part of Indiana, and the western part of Kentucky, and has an area 

 of 47,000 square miles. The coal is bituminous, and the aggregate thickness of 

 the coal-beds is about 40 feet. Indiana is celebrated for her block coal. There 

 are ten seams of coal in a vertical thickness of 600 feet in Illinois, and six of them 

 are from 2 and one-half to 6 feet each in thickness. 



161. The third coal-field in importance is the larger one, and occupies parts 

 of Iowa, Missouri, Kansas, Nebraska, Arkansas, and Texas, and has an area of 

 80,000 square miles. The coal is all bituminous. The western part of Missouri 

 and eastern part of Kansas bear coal in abundance. The Coal Measures are the 

 lowest Group of rocks exposed in Kansas, and have a thickness of 2,000 feet. 

 There are 22 seams of coal, varying in thickness from a few inches to seven feet. 

 Ten of them are more than a foot each in thickness. The coal in Arkansas is 

 excellent. 



162. The fourth coal-field is in Michigan, and occupies about 6,700 square 

 miles, with a thickness of about 125 feet. The coal is bituminous, and consists of 

 one bed from 3 to 5 feet in thickness throughout the whole shallow basin, being 

 thinnest near the border. Toward the central axis of the basin there are 2 or 3 thin 

 seams in close proximity to the main seam. The shales are well stocked with 

 fern-leaves and other terrestrial vegetation. There is a small area in Rhode Island 

 and Massachusetts of about 1,000 square miles, having a thickness of 6,500 feet, 

 but possessing no valuable coal-seam. The basin has suffered by the metamorphism 

 of the rocks and plication of the strata. The coal-seams have been changed to 

 anthracite, and are often somewhat wedge-shaped or of irregular thickness. 



163. The Coal Measures were deposited in basins, and must necessarily vary 

 much in thickness, the Group in Nova Scotia being thicker than elsewhere, and 

 the Group in Michigan thinner. The maximum thickness in Pennsylvania is 8,000 

 feet ; Ohio, 2,500 feet ; Tennessee, 2,500 feet ; Western Kentucky, 3,500 feet ; 

 Indiana, 1,000 feet, and Missouri, 2,000 feet. The Group is frequently separated into 

 an upper and lower series by the intervention of a conglomerate, and sometimes more 

 than one conglomerate exists in the Group. Marine vegetation abounds at some 

 localities, and land or marsh plants are distributed throughout the shales, sand- 

 stones, and coal. Coal was formed from plants which grew in swamps, marshes, 

 and open seas, and, where valuable, it is quite free from sediment, such as would 

 have accompanied much disturbance of the water. The beds usually rest on clay, 

 bearing Stigmaria and stumps of trees, and are followed by rocks bearing the leaves 

 of the vegetation of that era. The clay beneath the coal-beds is usually an argil- 

 laceous sediment, almost devoid of alkalies, and represents the ancient soil in which 

 the coal vegetation flourished, and apparently deprived it of the greater part of its 

 potash. This clay is usually excellent fire-clay. From the coal, as from modern 

 peat, the alkalies were almost entirely removed by the action of water. The 

 waters were fresh, brackish, and salt at different times and at different places. The 



