There is no other part of the geological series so obviously connected with national prosperity as the coal formation. While a country is new, the forests furnish an abundant supply of fuel; but in the course of a few years these are consumed. This country will soon be principally dependent upon its coal-mines for fuel, even for domestic purposes; and, in carrying on the great branches of national industry, such as the smelting and working of iron, and in the formation of steam for the purposes of manufacture and transportation, we are already mainly dependent upon mineral coal. A nation which does not possess an abundant supply of this mineral, or which does not use it, cannot long maintain a high degree of national prosperity.
In these inexhaustible masses of coal, accumulated ages before the existence of the human race, is a most obvious prospective arrangement for securing our happiness and improvement. And this arrangement embraces not only the accumulation of a combustible material in such abundance, but also its juxtaposition with an equally inexhaustible accumulation of iron ore, and the limestone which is necessary as a flux in the reduction of the ore. So bulky and heavy materials as coal and iron ore could neither of them have been transported to any considerable distance for the manufacture of iron; and without the manufacture of iron on a large scale, the present operations in manufactures and transportation could never have been entered upon. A large proportion of the iron furnaces in this country, and nearly all of them in Great Britain, employ mineral coal for fuel, and obtain their ore from the beds contained in the coal measures.
The fossils of the coal measures are almost entirely of vegetable origin, and are very abundant. They are seldom found in the coal-beds, but in the strata of shale immediately above or below the solid coal.
The Stigmaria ([Fig. 19]) is found most abundantly, and in a large proportion of cases to the exclusion of every other form, in the lower shales. It consisted of a large dome-shaped mass, often three or four feet in diameter, with trailing branches, or roots, spreading off horizontally to a distance of twenty feet. In a few instances tree ferns have been found, petrified in a horizontal position, and being apparently a mere continuation of the stigmaria. Hence the stigmaria has been supposed to be the base of the tall tree ferns, the leaves of which so abound in the upper shales. If this is not the case, there are no forms of the existing flora of the earth analogous to the stigmaria. It is always found in connection with the coal-beds of the carboniferous formation, and never with the coal-beds which sometimes occur in the later formations.
| Fig. 20. | Fig. 21. |
The tree ferns ([Fig. 20]) attained a height of fifty or sixty feet, and a diameter of four feet. They have received the name of Sigillaria in consequence of the seal-like impressions ([Fig. 21]) with which the surface is covered, and which are the scars left where the fronds have fallen off. These fronds (fern leaves) are the most abundant fossil of the series. They are distinguished by some peculiarity in form, as the Sphenopteris (wedge-shaped fern leaf), Pachypteris (thick fern leaf), &c. (Fig. [22.] and [23].)
There was another kind of Sigillaria ([Fig. 24]), in which the surface was fluted, and the markings are superficial, and occur on the ridges. It reached as great a size as the tree ferns, but to what general class of plants it belonged is still doubtful.