THE CARBONIFEROUS SYSTEM. ey 
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, 7. ¢., 
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. In 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been withdrawn to make the growth of a bed of coal on its sediment 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-erial 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 line 
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 where they 
are found forming nearly a homogeneous mass. Such instances occur in 
