116 GEOLOGY OF OHIO. 
annual, accumulation of the leaves, twigs, fruits, ete., 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 Qwhich 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. G. W. Binney, of Manchester, England, the theory has been advanced 
that coal was mainly formed from the spores (microspores and macrospores) of eryp- 
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 plants 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 (7. ¢., roots, stems and leaves), and we may infer that such has always been 
the case. 
