THE FOSSIL FLORA. 
161 
or formed like the sands of our sea shore, by the slow action of attri¬ 
tion upon rocks previously consolidated, hut that it had, probably, 
been produced by the ruin of crystalline rocks, so slightly coherent, 
as to have been unable to withstand the violent action of water, to 
which they had been exposed. The sandstones are all, more or less, 
micaceous, some of them Containing that mineral in large quantity ; 
where this is the case, and the plates are of considerable size, the 
stone is finely schistose. This is another proof that the materials 
forming the sandstone, had undergone little mechanical action previ¬ 
ous to disposition, or the fragile mica would have disappeared. 
In the series of beds, the coal itself forms, in bulk, a very inconsi¬ 
derable portion of the whole. Forty seams are enumerated, but the 
greater part of them are too thin to be worked to profit. 
The district has long been famous for producing coal of the finest 
quality, which has been extensively worked, and, up to the present 
period, the largest mining speculations in the kingdom, and, proba¬ 
bly, in the world, are carried on within it. This being the case, it 
has become a matter of great economical importance, to define, as 
nearly as possible, each separate bed in the series, and this has been 
done with great minuteness. It is the universal belief of those best 
practically acquainted with the subject, that even the thinner betls of 
coal, when not cut off by the rise of the strata to the surface, or by 
some fault, are spread out over the whole area of the formation. 
Whether this be the case or not with all the seams, we shall not stop 
to enquire; but the two beds known as the High and Low Main 
Seams, from their not only being the thickest, but as affording, in 
their whole mass, coal of fine quality, have been worked for centu¬ 
ries, and are known over a space, in the first instance, of more than 
eighty, and in the second, of two hundred miles square. 
In studying the Carboniferous formation generally, with reference 
to the circumstances under which its different members have been 
deposited, nothing is more singular than the sudden change in the 
nature of the beds composing it, and the clearly defined line by 
which these beds are separated from each other; this is most parti- 
ticularly striking in the lower portion, where a thick stratum of Car¬ 
bonate of Lime will be seen to terminate abruptly, and be immedi¬ 
ately succeeded by a bed of entirely mechanical origin, and of a 
composition so opposite, as to contain scarcely any calcareous matter 
whatever. Nor is the difference of the nature of the two beds more 
striking, than the difference ol their imbedded organic remains; 
whilst those of the limestone are almost exclusively of marine ani¬ 
mals, the sandstones very rarely contain fossils a! all; and these, 
M 
