COAL MEASURES, 



253 



teet. The red variety predominates, and especial- 

 ly towards the base of the series. Resting upon 

 these are massive strata of very coarse quartzose 

 conglomerates and sandstone, which, in a thickness 

 of a few hundred feet, generally constitute the 

 verge or summit of the mountain table-land. Upon 

 these beds again repose the bituminous coal meas- 

 ures, consisting of white sandstones, very analo- 

 gous to some of those above mentioned, intermin- 

 gled with other varieties of the rock, more argilla- 

 ceous, and with yellowish, gray, pink, and even red 

 sandstones in almost endless alternation. What 

 strongly characterizes this whole class of deposites 

 is the disproportionate amount of quartz or sand- 

 stones, and the paucity of slates and shales asso- 

 ciated with the coal. The coal-seams are usually 

 first met with soon after we pass the eastern verge 

 of the plateau, and here the coal measures are 

 mostly sandstones. Farther west we find the rocks 

 becoming more argillaceous, enclosing their beds 

 of soft shale and clay, and their irregular bands of 

 limestone. The secondary rocks abound both in 

 salt and gypsum. 



The above sections will serve to illustrate the 

 nature of the rocks and beds which contain coal in 

 the United States, and may answer as a useful 

 guide to such as are about to explore in search of 

 this mineral. They clearly show that there is a 

 very great similarity in the formations containing 

 it ; and a practical geologist can generally arrive at 

 a very safe conclusion, whether the mineral may be 

 sought with confidence, or the contrary, in any giv- 

 en locality.* 



Millstone Grit and Shale. — This is the uppermost 

 of the secondary rocks, and is composed of angu- 

 lar fragments of quartz and feldspar, held together 

 by a hard argillaceous cement. The shale differs 



* For an account of the extent of American Coal-fields, see 

 the chapter on the "Mineral Resources of the United States." 



