244 ANTHRACITE COAL MEASURES. 
then have the coal as it actually exists, both upon 
the hills and in the valleys, forming at times a por- 
tion of the strata of the mountains, and only occa- 
sionally lying in a basin form between the ridges. 
Lying nearer the transition and primitive rocks, 
w^hich are now universally allowed to have been 
once either in a melted state by fire, or subjected 
to great heat when the superincumbent strata were 
upraised, forming hills and mountains, we thus ac- 
count for the fact that the coal of these strata 
contain no bitumen, it having been dissipated by 
the heat, while the bituminous coal measure lying 
higher up in the series escaped its effects. To the 
same purpose Dr. Hildreth remarks, that "there 
seems to have been three different deposites of 
coal throughout the main coal-region of the west. 
After the vegetable materials which form the coal- 
bed were deposited or buried under the superincum- j 
bent strata, it would seem that a strong degree of ' 
heat had been apphed, in addition to the pressure, 
before they could assume their present biluminized 
appearance. As we approach the coal-beds in the 
transition and primitive rocks, the evidences of heat 
are still more* apparent, removing from the anthra- 
cite beds all, or nearly all, their bituminous con- 
tents, and in the primitive changing anthracite into 
graphite or plumbago, which is almost pure car- 
bon. It would appear that we cannot reasonably 
doubt the action of heat on these coals, for the 
plumbago is evidently a coal, changed by heat into 
its present semi-metallic appearance, and it is often 
produced in the furnaces of the arts by the action 
of heat upon carbon." The theory that anthracite, 
graphite, and bituminous coals are all of vegetable 
origin, and owe their present form to different de- 
grees of heat and pressure, is in the highest degree 
rational, and well supported by facts. 
