THE CARBONIFEROUS SYSTEM. 117 



the area of the upper coals, but had they ever reached as far as the 

 lower ones they would certainly be found elsewhere than they are, i. c, 

 only in the center of the basin. In the western coal fields we find that 

 the subsidence was progressive in one or another direction, the upper 

 coal seams then reaching in this direction far beyond the lower. la the 

 Ohio portion of the Alleghany coal field, however, the basin seems to 

 have narrowed as it deepened. That intervals of elevation alternated 

 with those of subsidence seems proven by the fact that beds of fire-clay 

 and coal sometimes rest directly upon limestones which must have been 

 deposited in somewhat clear and deep water. This water must have 

 been withdrawn to make the growth of a bed of coal on its sediment 

 possible. Proofs of greater elevations are also not wanting in the Coal 

 Measures, such, for instance, as is furnished by the following case, re- 

 ported by Mr. M. C. Read : In Clarke township, Coshocton county, is a 

 channel, now filled with sandstone, 280 feet deep. This cuts out in a 

 narrow belt all the lower coals from No. 5 down. Complete sections in 

 the vicinity show the coal seams to be regular and undisturbed on either 

 side. This is the result of sub-serial erosion, and proves that during the 

 Coal Measure epoch this region was elevated several hundred feet above 

 the sea level. Thus we see that our Coal Measures form the record of 

 a subsidence of the great geosynclinal lying between the Blue Ridge 

 and the Cincinnati axis, a subsidence which carried the central portion 

 of the trough down at least 2,000 feet. This would have formed here a 

 deep synclinal valley, but that, being a comparatively narrow trough 

 and receiving the drainage of a continent lying north and east, it was 

 filled nearly as fast as formed. That the sinking was unequal we learn 

 from the unequal distribution of the limestones, which are the most dis- 

 tinct marks of the reach and continuance of the successive submerg- 

 ences. The great limestone associated with the Pittsburgh coal, for 

 instance, occupies only the central portion of the basin, and thins out 

 both east and west, while some of the lower limestones have their lire 

 of greatest development quite within our State and are unknown in 

 Pennsylvania. The same thing is taught by the coal strata, some of 

 which are quite local ; others are very extensive, but none cover the 

 whole breadth of the basin. But the best proof of unequal subsidence 

 that we find in the Coal Measures is afforded by the great variation 

 which is observable in the interval which separates the different seams 

 in the series (examples of which will be given hereafter) and in the 

 splitting up of our coal seams into two or more subordinate seams in 

 their extension in one or another direction from localities wh^re thev 

 are found forming nearly a homogeneous mass. Such instances occur in 



