Page:The American Cyclopædia (1879) Volume IV.djvu/743

Rh end of a gangway and cannel at the other, and this in the whole thickness of the bed. The analysis of caking coal fails to show any difference even in the proportion of the constituent elements from that of some kinds of dry or non-caking coal. Indeed, the character of the coal, where closely examined, is constantly variable in the same bed at the same mine, even upon the same square foot of matter, as recognized from specimens when taken at the same place from the roof to the bottom of the bank, although little or no difference is observed in a large quantity as it comes from the mine.

In order to understand more easily the distribution of the combustible minerals, especially coal, it is convenient to have for reference a tabular section of the American geological divisions from the earliest times till now, as they have been recognized by science. The following brief review of the formations, from the lowest or oldest to those of our own time, has special reference to such evidence as they show of coal or any combustible mineral resembling it.—No trace of remains of either plants or animals has been positively recognized in the lowest formations of the earth, which, composed generally, at least, of crystalline metamorphic rocks, are considered as the result of the cooling of the surface of our planet, which was originally in a state of fusion or of vapor. The archæan rocks, also called primitive rocks, are therefore the only ones universally formed, all the others depending upon local abrasions for their materials, which, transported and deposited mostly by water, are local in their distribution. Animal life is now, and must have been from the beginning, dependent upon vegetable life as the only source of its food. The first traces of organic remains should for this reason represent plants. The primitive or archæan formations have deposits of graphite or plumbago, a matter essentially composed of carbon. It is not known as yet how this matter has been produced or whence

it is derived. It has been and may be ascribed to vegetable and animal life, represented at its beginning by beings of very simple soft texture, like the confervoidal filaments which at our time live in thermal springs, filling basins of water of the temperature of the boiling point mixed with animalcules or infusoria. The remains of these plants and animals could not have been preserved, or at least could not be discovered, in the crystalline matter of the primitive rocks. The presence of graphite in the carboniferous strata of Rhode Island, and the close likeness of some beds of anthracite of this basin, which in some of its veins is scarcely distinguishable from graphite, point to vegetables for the origin of this substance. For even the hardest layers of anthracite or graphite of Rhode Island bear well preserved remains of plants of the carboniferous period, and evidently their carbon has been derived from vegetable life. The graphite of the primitive rocks, however, like the crystalline matter, granite, mica, hornblende, syenite, &c., may be due to some as yet unknown combinations of the primitive matter of our globe. The primordial or Cambrian period is subdivided into two epochs: the upper, called the Potsdam, and the lower, the Acadian epoch. In this last formation the first remains demonstrating vegetable life appear in some fucoids or marine plants of undefined forms. They become more numerous and more distinct in the Potsdam sandstone, in which large species of algæ have been obtained and described. Their size indicates already a high degree of organized life. In the Canadian period, especially in the calciferous limestone which constitutes its lowest division, these fucoidal remains increase in abundance, representing many more species, and the rocks where the remains are imbedded are often discolored by what appears to be an impregnation of mineral oil. The matter is however very sparingly distributed. But in the Trenton period, from its lowest division, the Trenton limestone, to the Cincinnati, its upper epoch, the marine vegetation is evidenced not only in an abundance of petrified plants, but in local deposits of mineral oil, especially found in connection with a predominance of fucoidal remains, which thus attest one of the wise purposes of their life in the great plan of nature. The Hudson shales give mineral oil sparingly; the Cincinnati limestone has yielded it abundantly; the black Utica shale of the same period has sometimes from 12 to 20 per cent. of mineral oil; but no trace of coal has been found in the rocks of the Trenton period. The three divisions of the Niagara period, the Niagara, the Clinton, and the Medina, have also, in their shales, limestones, and sandstones, a prodigious abundance, in some localities at least, of marine plants. In Pennsylvania the Clinton ferruginous red shale is covered over wide surfaces with these kinds of vegetable remains, together with a proportionate number of remains of