344 THE POPULAR SCIENCE MONTHLY. 



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 de- 

 composition 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, serai- 

 anthracite, semi-bituminous, and the many different varieties of bitu- 

 minous, is supposed to be owing to the different degrees of progress 

 made in the process of softening and carbonization, and to there hav- 

 ing 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 subse- 

 quently 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 man- 

 ner the theories of the formation of our fossil fuels, and, while difficul- 

 ties may suggest themselves to the reader, still that the}'^ 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 



