180 



NATUKAL SCIENCE NEWS. 



View of the Petrified Forest, Apache County, Arizona. 



of an inch in diameter, were in that 

 time represented by trees from one 

 to three feet in diameter and from 

 40 to 100 feet in height. This mass 

 of vegetation, including more than 

 500 different species, was constant- 

 ly growing, falling and decaying, 

 each succeeding growth forming a 

 still richer bed for the vegetation 

 to follow. 



If the theory propounded by La- 

 place is correct, our earth was at 

 one time a ball of liquid fire. Cool- 

 ing and condensation progressed 

 from the surface toward the cen- 

 ter. Contraction of the earth's 

 crust necessarily followed, and 

 vast areas of land sank and were 

 covered by the waters. This pro- 

 cess was still going on during the 

 carboniferous age. The submer- 

 gence of a bed of this incipient 

 coal meant the cessation, for a 

 time, of vegetable growth from its 

 surface. That surface was cover- 

 ed instead by the sand, mud and 

 gravel washed over it by the waves, 

 by the drift from higher levels, and 

 by the limestone deposits swept up 

 to it from the sea. When contrac- 

 tion ceased for a time and the 

 earth's crust again became stable, 

 the waters began to recede, leav- 

 ing behind them great wastes of 

 mud and sand. And, following 

 this slow recession to the sea, ve- 

 getation crept once more over the 

 surface ot the land, the soil grew 



rich with the products of decay, 

 and plant life reigned and rioted 

 anew. But cooling and contrac- 

 tion of the earth's body were go- 

 ing continuously on, and submer- 

 gence followed again and again, 

 each bed of vegetable matter, thick 

 or shallow, being covered in turn 

 by its layers of sand and silt. 



In this submergence and burial 

 of the deposits of the coal era we 

 find all the conditions necessary 

 for the transformation of vegetable 

 matter into coal. Only from one- 

 ninth to one-sixteenth of the mass 

 of vegetable matter subjected to 

 this heat and pressure was retained 

 in the form of coal. This was 

 largely carbon, the hydrogen and 

 oxygen having been expelled. As 

 we have already seen, the anthra- 

 cite coal contains a much larger 

 precentage of carbon than does the 

 bituminous, and a much less quant- 

 ity of volatile matter. Of the im- 

 mense coal areas in the United 

 States only an extremely small 

 percentage are of the anthracite 

 variety, and these all lie in the 

 State of Pennsylvania, east of the 

 Allegheny Mountains, with the ex- 

 ception of a small field in Rhode 

 Island. It is not thought that the 

 vegetable life which entered into 

 one class differed in any material 

 respect from that which entered 

 into the other. 



The presumption is natural, if 



not conclusive, that prior to the 

 close of the carboniferous age all 

 the coal deposits had been bitu- 

 minous in character, but that the 

 violent movement of the earth's 

 crust at the time of the Appalach- 

 ian revolution, the enormous pres- 

 sure and intense heat, were suffi- 

 cient to expel a large portion of 

 the volatile matter from the bitu- 

 minous coal beds, and otherwise 

 change their character into what 

 we now class as anthracite. In the 

 slate strata immediately overlying 

 each coal seam, it is common to 

 find the impressions of twigs, nuts, 

 seeds, leaves, the most delicate fern 

 tracery, and the trunks of great 

 trees mashed flat between the 

 layers; while in the softer beds of 

 cannel coal, whole trees have been 

 found, roots, trunks, branches, 

 leaves, seeds, and all transformed 

 into like material with that by 

 which they were surrounded. One 

 of the results of the violent distur- 

 bances of the earth's crust already 

 noted was to leave great rents in it 

 across the lines of strata. These 

 rents are known geologically as fis- 

 sures. They have faces which are 

 either parallel or inclose a wedge 

 shaped cavity. Sometimes igneous 

 rock from the molten mass below 

 was forced up into these openings; 

 sometimes the cavities were filled 

 with drift and rock fragments from 

 the surface. In either case the 



