Page:Popular Science Monthly Volume 14.djvu/358

344 slate, according to the preponderance of the mud which was with it. Thus we can understand one of the causes why some regions have more seams than others.

When the deposit was covered up, as before explained, a gradual decomposition took place, which consisted in an evolution of a portion of the carbon, and most of the hydrogen and oxygen, in the form of water and gases from the woody tissue, leaving a larger and larger percentage of the carbon of the plant behind, while the increased pressure of the accumulating strata above served to compress and solidify the mass. But before this solidification took place, as Liebig has proved, by direct experiment in the process of slow decomposition of vegetable matter in water, a softening occurred, and it is to this that we must ascribe the fact that no delicate fossils are ever found in the coal itself, as the tissue and form were destroyed by the softening and subsequent pressure, though cases are met with where solid trunks of trees have resisted this softening process, and are found standing erect in the seams while their roots are plainly traced in the clay-slate below. In the slates above and below, which, it must be remembered, were originally soft, plastic mud, the plant-impressions, however, are as sharp and clear as though they had been sketched with an artist's pencil.

The formation of different kinds of coals, such as anthracite, semi-anthracite, semi-bituminous, and the many different varieties of bituminous, is supposed to be owing to the different degrees of progress made in the process of softening and carbonization, and to there having been freer escape for the gaseous constituents in some cases than in others. Chemists have actually converted vegetable matter into coal of all degrees of hardness, and possessing all the various qualities of that formed by Nature, and observation and their labors seem to show that all coal was first formed of the bituminous variety, and that anthracite is the result of igneous action to which it was subsequently subjected (MacFarlane). When this change has been carried still further, the result is plumbago, or black-lead.

I have thus endeavored to set forth in a plain, comprehensible manner the theories of the formation of our fossil fuels, and, while difficulties may suggest themselves to the reader, still that they are derived from the vegetable kingdom admits of no doubt, this being one of the well-established facts of geology.

There is one more benefit that coal has been the cause of bestowing upon mankind that is as striking as all those previously set forth, to which I would call attention before closing. The Bible tells us that the beasts of the field and the fowls of the air were not created until after the earth brought forth grass, the herb yielding seed, and the fruit-tree yielding fruit after his kind; and with the aid of science we can see a reason for this. It has been stated that one of the requisites for the vegetation of the coal era to flourish as it did was that the atmosphere should be charged with a compound of carbon and oxygen known as