COAL MEASURES. 
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4. Sandstones of different qualities, often in very 
thin laminae. 
6. Coal, forming numerous beds or layers in the 
sandstone. 
6. Sandstone, called millstone grit, because used 
for millstones, containing vegetable remains. 
7. Calcareous slate or shale, from 300 to 600 feet 
thick. 
8. Mountain or primitive limestone, the lower 
strata containing marine animal remains, the upper 
fresh- water deposites, with terrestrial vegetables. 
4. Coalhrook Dale Coal-bed, 
Dr. Mantell, in his " Wonders of Geology," gives 
the following account of this coal-field : It is situ- 
ated on the east side of the range of transition rocks 
forming the Wrekin and Wenlock Edge, and the 
coal strata are superimposed on millstone grit. Beds 
of ironstone occur, abounding in nodules, with or- 
ganic remains. This coal-field is remarkable for 
the dislocated and shortened state of the strata, and 
the intrusion of volcanic rocks, which do not ap- 
pear as dikes, as in the fissures of the beds, but rise 
up in mounds or protuberances. The walls of the 
fissures are in some instances several yards apart, 
the intervals being filled with debris. Beds con- 
taining marine shells alternate with others abound- 
ing in fresh-water shells and land-plants. The se- 
ries of strata forming this carboniferous accumula- 
tion consists of sandstone, indurated clay, slate 
clay, and coal. A pit sunk in Madely Colliery, in a 
depth of 730 feet, passed through 86 beds of alterna- 
ting quartzose sandstone, clay porphyry, coal, and 
indurated clay, containing argillaceous ironstone in 
nodules. The sandstones of Coalhrook Dale are 
fine grained and micaceous, and some beds are pen- 
etrated by petroleum, which at Coalport escapes 
from the surface in a tar-spring ; bitumen also oc- 
curs in some of the shales. Plants, shells, and 
