COAL MEASUBES. 7 



miners term a "horseback," and pass below the coal without showing 

 any indications of the existence of a workable seam. 



On the eastern borders of the State, in the counties adjacent to the 

 Wabash river, several borings have been made that failed to hud any 

 coal thick enough to be of any value, and it is possible that there are 

 some local areas where there are no heavy beds developed ; but this is 

 a point as yet unsettled, and only to be determined by careful experi- 

 ments at many different localities. It is certainly not a well recognized 

 principle in geology that the central portion of a coal field should be 

 barren, and only the borders productive, and there is no good reason 

 to suppose that the Illinois coal basin will prove an exceptional case 

 in this respect. Sir Chaiules Lyell suggests, in his " Principles of 

 Geology," that the facts seem to " imply the existence, during the car- 

 boniferous epoch, of islands, instead of an extensive continent, in the 

 area where the coal was found." If we accept this as probably oue of 

 the prevailing conditions of the coal-producing epoch, we must expect 

 to find certain areas in the coal fields where the surface was not ele- 

 vated above the ocean level long enough to yield a forest growth suf- 

 ficient, when again submerged, to form a coal seam, and consequently 

 local areas of greater or less extent where no workable coal can be 

 found. 



It is now a very generally accepted proposition that the vegetable 

 matter necessary to the production of a coal seam grew upon the spot 

 where the coal is found, and was not, as formerly supposed, drifted 

 from an adjacent shore into the ocean's bed, where it was finally cov- 

 ered by sediments and transformed into bituminous coal through the 

 slow chemical processes of succeeding ages. Hence coal would only 

 be found where the conditions requisite for a dense growth of tropical 

 plants prevailed, and near the ocean level where the land was liable to 

 submergence. All the remains of animal life found in the limestones 

 and calcareous and bituminous shales that are associated with the coal 

 in this State are of marine origin, showing conclusively that the beds 

 from which they come have been formed beneath the ocean, and not 

 under fresh water, as formerly supposed, and hence our present coal 

 fields must have been low peaty and boggy lands adjacent to the sea 

 shore, and subject to frequent and long continued submergeucies,. 

 during which the sandstones, shales and limestones separating the 

 various seams of coal were deposited, inclosing the remains of fishes, 

 molluscs and other marine organisms with which the ocean was filled 

 at that period. 



In defining the boundaries of the coal field on the State map, we 

 have been compelled to rely mainly on the reports of borings for the 

 counties of Kankakee and Iroquois, as there are few or no natural out- 

 crops of the strata along the borders of the coal area in these counties, 



