II. ORIGIN OF THE CARBONIFEROUS ROCKS.—The strata comprised within the coal-measures are variously estimated; being, in some instances, about eleven thousand feet in thickness; in other cases, of much greater depth; and of this mass of matter, the coal itself does not occupy more than a maximum average of one hundred and fifty feet. The shales consist of thin beds of mud, washed down by the rivers from the neighboring heights, and would appear to have formed the soil on which subsisted a rank vegetation; the impressions of plants, roots, and trunks of trees being still found in a standing position. It is from these bands of mudstone that the best specimens of the flora of the period are derived; every thin splitting presenting the most entire and beautifully-preserved figures of fronds and stems. The ironstone is usually mixed up or associated with the shales, and consists, like them, of comparatively thin beds of ferruginous clay. The sandstones, of which the greater proportion of the mass consists, have clearly resulted in the continued action of the same causes that produced the old red deposit of the anterior period. But the two remarkable products of the age are the calcareous and coaly strata, which give character to the system as well as the epoch in which they were formed; the one showing a sudden development of carbonate of lime, and the other an increase of vegetable matter, whose enduring monuments point them out as the most striking cotemporaneous and co-extensive formations on the surface of the globe, or connected with the history of our planet. The bituminous products of the Silurian period, if the anachronism may be pardoned, are but as the gleanings after the full harvest.