COAt MEASURES- 
253 
feet. 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 ^f 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.'* 
