168 NEW YORK STATE MUSEUM 



100 feet, and are separated from each other by masses of rock from 

 10 or 20 to 200 or 300 feet thick, and are mined in various ways 

 according to their situation. 



Geologic investigation in all coal regions has led to the conclu- 

 sion that the strata of coal are composed of vegetable matter, 

 which during the Carboniferous epoch appears to have reached 

 an enormous and luxuriant growth, and formed vast accumula- 

 tions, which after being buried under the marine sediments of 

 clay and sand which now form the shales and sandstones over 

 them, underwent chemical changes which transformed them 

 to their present condition. The proofs of this are found in the 

 fact that the rocks above and below the coal seams are filled 

 with vegetable remains, leaves, stems, roots, etc.; the trunks of 

 the trees being in some places found still erect and standing 

 upon their roots, but converted into coal; and that even the coal 

 itself, though in most cases it is solidified into one mass so as to 

 show no organic structure, displays in other instances, under the 

 microscope, all the structure of wood; the cells, the ducts through 

 which the sap once circulated, and even minute markings by 

 which it can be determined whether the wood belonged to one 

 or another general class of trees. 



The vegetable origin of all coal is well established; but the 

 mode in which great accumulations of it were made, over such 

 vast areas, is yet an obscure question. A single bed of coal, that 

 called the Pittsburgh seam, is known to extend over no less than 

 14,000 square miles, with a usual thickness of from four to ten 

 feet. Other layers, though less in extent, are much greater in 

 thickness, reaching even 100 feet. The prevailing opinion is that 

 it grew in enormous morasses or swampy tracts, resembling on 

 a larger scale the Great Dismal swamp, or the Okefinokee swamp 

 of Georgia, in which the annual fair of leaves, branches, and 

 trunks through a long period of time formed thick peaty masses, 

 which, being submerged under the sea and covered with sedi- 

 ments, became the vast deposits of fossil fuel which are now of 

 so great importance. 



