SECONDARY ROCKS. 
8& 
CHAPTER VIII. 
SECONDARY ROCKS. 
Carboniferous Group, 
Division of Secondary Rocks. — Coal Measures. — Vegetation of 
Carboniferous Formation. — Lower Secondary Rocks, how 
Divided.— Millstone Grit — Its Mineral Contents.— Carbonifer- 
ous Limestone — Its Extent in this Country. — Section of Coal 
Measures in England. — South Gloucestershire Coal-basin. 
—Coal-fields of Derbyshire.— Coalbrook Dale. — Coal Meas- 
ures of North America. 
The secondary rocks have been divided into the 
upper secondary and the lower secondary. The 
lower secondary are equivalent to the coal formation^ 
or, as it is sometimes called, coal measures. These 
are composed of various beds of sandstone, shale 
or slate, and coal, irregularly interstratified, and in 
some places intermixed with conglomerates or 
clay ; the whole showing a mechanical origin. Coal 
measures abound in vegetable remains, and the coal 
itself is almost universally referred to a vegetable 
origin, being considered the accumulation of an im- 
mense mass of plants. These have been distribu- 
ted either by the agency of fresh or salt water 
floods, over areas of greater or less extent, upon a 
previously deposited surface of sand or argillaceous 
mud, which afterward has been converted into 
slate. After the distribution of these vegetables, 
other sands or mud were accumulated upon them ; 
and this operation, in some places, has been appa- 
rently repeated several times. 
The vegetables which enter into the composition 
of coal appear to belong to the lowest order chiefly, 
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