THE CARBONIFEROUS PERIODS 501 



grew, is to be found (1) in the basin-shaped seams which are often 

 thickest in the center and thin out to black shale at the edges ; (2) in 

 the remains of aquatic animals in the midst of the coal; (3) in the 

 roots of trees embedded in the underclay in the position in which they 

 grew; and (4) in the purity of the coal. If the coal was formed from 

 vegetation that had drifted together, it would contain sand or mud in 

 appreciable amounts. It is not unusual, however, to find coal with 

 no more impurities (ash) than the wood from which it was derived 

 would have contained. The nearly uniform thickness of the coal 

 beds over hundreds of square miles is also offered as an objection to 

 the theory that the vegetable matter was drifted together. 



(3) How it was Changed to Coal and what Varieties Resulted. — The 

 principal varieties of coal are peat, the partially decayed vegetation 

 of swamps ; lignite or brown coal ; bituminous or soft coal ; and an- 

 thracite or hard coal. All have been derived from peat, lignite being 

 the second stage, bituminous the third, and anthracite the fourth. 

 The last stage is graphite, in which all the volatile constituents have 

 disappeared and pure carbon only remains. Anthracite coal occurs 

 in regions where the strata have been much folded and faulted. It 

 is therefore generally believed 1 that the heat and pressure of dynamic 

 action are essential processes in coalifaction or bituminization. It 

 has also been suggested that in regions of great folding the fractures 

 which have been produced facilitate the escape of gases from coal and 

 thus hasten the process. In Rhode Island dynamic metamorphism 

 has been so intense that the coal has gone beyond the anthracite stage 

 and contains so much graphite as to be of little value. In Colorado 

 and elsewhere bituminous coal has been converted to anthracite 

 where cut by dikes, and in Mexico coal has been baked to graphite by 

 heat. Such graphite is of great value in the manufacture of lead 

 pencils, Some varieties of coal result from the kind of vegetation 

 of which it is composed. Cannel coal, for example, is made almost 

 wholly of the spores of Carboniferous plants. 



Conditions Favoring Coal Formation in the Pennsylvanian. — To 

 understand the great accumulation of coal during the Pennsylvanian 

 one must picture to himself the conditions at that time. The land 

 appears to have been low, and sluggish streams meandered through 

 extensive fresh-water marshes. The great inland seas, shut off on 

 the east by the continent of Appalachia, were bordered by wide 



1 Prof. E. C. Jeffrey offers evidence to show that the varieties of coal depend largely upon 

 'heir composition. 



