E eolation of the Earth. 89 



fore, composed entirely, or almost entirely, of organic re- 

 mains. Coal, for example, which exists in such vast quan- 

 tities in many portions of our country, especially in Penn- 

 sylvania, is almost pure carbon in its chemical composition, 

 is deposited in layers like the sedimentary rocks, and is 

 really so much vegetation which has been hardened into 

 stone. During the Carboniferous era when the material of 

 which the coal-beds are formed was produced, the atmos- 

 phere was densely loaded with carbonic acid gas and aque- 

 ous vapors. Nearly one-half of the earth's crust is said to 

 be composed of oxygen and other materials which original- 

 ly existed in a gaseous condition. The purification of the 

 atmosphere from these vapors and gases, constituted another 

 example of differentiation and integration illustrative of 

 the law of evolution. 



Beneath a section of a coal-seam, you may often trace 

 the blackened lines of the roots of former trees, stretching 

 down into the lower stratum of rock or clay. Originally, 

 this rock or clay was a bed of soil, in which grew a luxur- 

 iant vegetation. Some of the mines in Nova Scotia pre- 

 sent as many as seven different layers of forests, which 

 have grown during the lapse of ages, each in turn above 

 others, until at last, all have been consolidated into coal. 

 Examine a thin piece of soft coal under a microscope, and 

 you may often find millions of little sporangia, or seed ves- 

 sels, — the product of a kind of moss which was pressed 

 into beds of peat, and then hardened into coal. In the peat 

 bogs of Ireland, in our own country, and elsewhere, this 

 process is still going on, though much more slowly than 

 during the Carboniferous period, laying, possibly, the foun- 

 dations of coal-beds for the use of future generations. In 

 the rock underneath or above the coal-beds, you may some- 

 times discover fossil forms of the trees and larger plants 

 out of which coal is formed. Some of these trees are of 

 curious conformation, having leaves growing directly from 

 their trunks, instead of on limbs and branches. The scars 

 left by these leaves when they dropped off are still visible 

 in the petrifications. 



Not only are stratified rocks thus formed out of vegetable 

 material, they are also built up out of animal remains. 

 Last summer, during my annual vacation, I spent a delight- 

 ful fortnight with a congenial comrade, walking through 

 the beautiful hill-country of Western Massachusetts and 



