46 GEOLOGICAL SURVEY 
Huntsville, Ala. some little mounts occur, composed almost entirely of this rock, which Dr. New- 
man informed me are 1000 feet high. In general it assumes its greatest thickness towards the 
South. It is in this rock that the great caves of the West are found. 
T'wo characteristic fossil corals are abundant in this formation, Stylina Perroni and the Archi- 
mides of Lesueur. Besides these, innumerable crinoideans of the genus Pentremites are found 
studding the weathered surface of the rocks. 
Next above this limestone we have beds of sandstone and conglomerate, that in the Western and 
South-western States vary from 50 to 200 feet in thickness. The beds furnish mill-stones in every 
country where they occur, and hence the name mill-stone grit, by which they are every where 
known. The sandstone is also equally noted for furnishing good fire-proof stone for iron furnaces 
and similar purposes. 
Resting immediately upon these, we find the Coal Measures, which consist of beds of shale, 
clay, coal, and sandstone, and from which this system derives its name and importance. 'The 
coal is found in seams, alternating with beds of shale and sandstone, and varying in thickness 
from an inch to many yards. 
In some of the English coal-fields the number of these seams amounts to seventy-five, making 
a total thickness of 150 feet of coal. 
The great coal-field on the West of the Alleghany Mountains, extending from Pennsylvania to 
Alabama, must be 800 miles long and 200 miles wide at its greatest width. While the coal-field of 
Illinois must equal (Dr. Owen) in extent the whole of England. 'There are from six to ten seams 
of coal in these coal fields. 
Although coal is found enclosed between beds of sandstone, in general it is found resting on 
beds of clay, called under-clay, which is more or less mixed with carbonaceous matter, and con- 
taining Stigmarie, with rootlets extending in all directions through the clay. 'The coal presents 
a jointed structure, which causes it to break into prismatic fragments. The upper surface of the 
coal is often covered with vegetable impressions and even charcoal. The shale which overlies the 
coal is laminated, and between the lamine impressions of ferns are found in great abundance and 
in fine preservation. Both in the shale and overlying sandstones fossils of the genera Sigillaria, 
Stigmaria, Calamites, and Lepidodendron, are quite common. 
It is now universally admitted that coal is of vegetable origim—wood, &c. carbonized under great 
pressure. Whenever the carbonization of the vegetable matter took place under circumstances 
that prevented the escape of the gases the result is bituminous coal, but where they were allowed 
to escape, anthracite is produced. We have abundant proof of this wherever the coal measures 
are much fractured—as, for instance, along the anticlinal axis of the Alleghany Mountains, where 
the anthracite of Pennsylvania occurs, and in the New Red sandstone of North Carolina, where 
the bituminous coal is converted into anthracite in the vicinity of the trap dykes, by which it is 
intersected. 
The structure of wood may be detected in coal by the aid of the microscope, and it appears that 
coniferous trees formed no inconsiderable portion of the mass. Drifted wood, accumulating in 
estuaries and at the bottom of seas of limited extent, has been proposed to account for the carbo- 
naceous matter of coal; but the perfect state in which fossil plants of the most delicate structure 
are preserved in the overlying shale, seems, in most cases at least, to forbid this, for it is difficult 
