116 GEOLOGY OF OHIO. 



annual, accumulation of the leaves, twigs, fruits, etc., of the plants 

 which covered the coal marshes. This we learn from a careful micro- 

 scopic study of the coal itself.* Hence the coal beds, though of insig- 

 nificant thickness as compared with the associated strata, probably rep- 

 resent long intervals of time. These intervals, however, ultimately 

 ended, and the peat bogs, the growth of which took place at or above 

 the water level, were submerged generally at considerable depth, for 

 we find them overlaid by sedimentary strata many feet in thickness. 

 Usually the water which flowed over them transported and deposited 

 clay or sand. When the change took place quietly the sediment was 

 fine, and we now find it as a clay shale;' when attended with more 

 violence the motion of the water was quicker, its transporting power 

 greater, and it spread thick sheets of coarse material sometimes over 

 large areas. Oftener than otherwise, this turbulent flood or rapid cur- 

 rent succeeded a period of quiet submergence, as we generally find 

 shales succeeding the coal, and this in turn overlaid by sandstone, this 

 sandstone locally cutting out the shale or coal, or both, and forming 

 what are known in miners' language as horsebacks, which are simply 

 beds of sand deposited in channels cut by water currents in the then 

 soft materials, now forming our beds of shale and coal. Where the 

 subsidence, greater than usual, resulted in the extension into the coal 

 basin of an arm of the sea, this quietly deposited calcareous sediments, 

 which now form limestones. In process of time the water basins in which 

 the sediments I have described — shales, sandstones, and limestones — 

 were deposited, were, sometimes by elevation, sometimes by filling up, 

 shallowed until they were again pools and marshes, where fire-clays and 

 beds of coal were again formed, again to be submerged. In this way 

 the whole 1,000 feet of our Coal Measures have been built up and form 

 a record of a subsidence along the center of the coal basin (which passes 

 near Pittsburgh) of more than 2,000 feet. That this subsidence was 

 local we learn from the fact that the upper coal beds occupy narrower 

 limits than the lower. Erosion may have done something to contract 



* By Mr. E. "\V. Binney, of Manchester, England, the theory has been advanced 

 that coal was mainly formed from the spores (microspores and macrospores) of cryp- 

 togamous plants, such as Lepidodendron, Sigillaria, etc. ; but a searching examination 

 of our coals has shown me that though sporangia and spores are common enough 

 in the coal beds, they make up no considerable portion of the mass. In all classes 

 of p'ants living at the present day the organs of fructification are insignificant in 

 volume as compared with the organs which belong to the vegetative system of the 

 plant (i. ( -.., roots, stems and leaves), and we may infer that such has always been 

 the case. 



