CARBONIFEROUS PERIOD. 327 



coal ; but these, as well as the rest, contain remains of roots [Stig- 

 marice), and often support still the old stumps. (Dawson.) 



The rock capping a coal bed may be of any kind, for the rocks 

 are the result of whatever circumstances succeeded ; but it is 

 common to find great numbers of fossil plants and fragments of 

 trees in the first stratum. 



The shaly beds often contain the ancient ferns spread out between 

 the layers with all the perfection they would have in an herba- 

 rium, and so abundantly that, however thin the shale be split, it 

 opens to view new impressions of plants. In the sandstone layers, 

 broken trunks of trees sometimes lie scattered through the beds. 

 Some of the logs in the Ohio Coal measures, described by Dr. Hil- 

 dreth, are 50 or 60 feet long and 3 feet in diameter. 



The thickness of the coal beds at times hardly exceeds that of 

 paper, and again is from 30 to 40 feet. The beds also vary in purity, 

 from coal with but 1 per cent, of earthy matter, to dark-colored 

 shales with only a trace of coal. The thickness is seldom over 8 

 feet, and the impurities ordinarily constitute from 7 to 15 per cent. 



The Pittsburg seam, at Pittsburg, Pa., is 8 feet thick. It borders 

 the Monongahela for a long distance, the black horizontal band 

 being a conspicuous object in the high shores. It may be traced, 

 according to Rogers, into Virginia and Ohio over an area at least 

 225 miles by 100 ; and even into Kentucky, according to Lesquereux. 

 But it varies in thickness, being 12 to 14 feet in the Cumberland 

 basin, 6 feet at Wheeling, 5 at Athens, Ohio, and on the Great 

 Kanawha ; farther south, at the Guyandotte, 2 to 3 feet. 



The "Mammoth Vein," as it is called, which is exposed to view 

 at Wilkesbarre, Pa., is 29J feet thick. It is nearly pure through- 

 out, although there are some black shaly layers 1 to 12 inches 

 thick. The same great bed is worked at Carbondale, Beaver Mea- 

 dows, Mauch Chunk, Tamaqua, Minersville, Shamokin, etc. 



At Pictou, in Nova Scotia, one of the coal beds has the extra- 

 ordinary thickness of 37J feet, and a second 22J feet. 



2. Structure of the Coal. — A bed of coal, even when purest, consists 

 of distinct layers. The layers are not usually separable, unless the 

 coal is quite impure from the presence of clay ; but they are 

 still distinct in alternating shades of black, and may be seen in 

 almost any hand specimen of the hardest anthracite, forming a 

 delicate, though faint, banding of the coal. 



In much of the bituminous coal of the Mississippi basin a cross- 

 fracture shows it to be made up of alternate laminae of black, 

 shining, compact bituminous coal, and a soft, pulverulent car- 

 bonaceous matter, much like common charcoal. 



