84. GEOLOGY OF OHIO. 
south-west toward the north-east is shown by the fact that the lime- 
stone is thickest at the south-west, and thins out to a feather edge to- 
ward the north-east; reaching as far as central Ohio and the south Hne of 
Pennsylvania, where the only portion of the Lower Carboniferous lime- 
stone found is the upper or Chester division. Fourth—In the third and 
last epoch of the Coal Measures the Carboniferous sea retreated and left 
a broad area of shallow water and dry land. The transition from marine 
to terrestrial conditions is recorded in the Conglomerate, while the Coal 
Measures accumulated in synclinal troughs, which gradually sank, with 
many oscillations, and were filled by the wash from the surrounding land 
in the form of sand and clay, now sandstones and shales; by beds of 
peat, now coal, which grew on the marshy surfaces; and by the lime- 
stones deposited from the inflowing sea during the periods of local sub- 
mergence. When sediments had accumulated in these basins to the 
depth of 2,000 to 3,000 feet, the continent was affected by great disturb- 
ances, recorded in the Alleghany Mountains, then raised. At this time 
nearly all the area between the Atlantic and the Mississippi was brought 
above the ocean level, where it has remained until the present time. 
THE WAVERLY GROUP. 
The Carboniferous system consists in Ohio, as almost every where else, 
of three great subdivisions: the Coal Measures above, the Conglomerate 
‘in the middle, and the Lower Carboniferous group below. The Lower Car- 
boniferous strata are the Lower Carboniferous limestone, and those desig- 
nated by the geologists of the former Survey as the Waverly sandstone series, 
from the outcrops which contain the famous building stone in the vicinity 
of the town of Waverly, Pike county. No effort was made by those who 
first described this formation to determine its precise geological age, or 
its relations to the rocks of New York and Pennsylvania. Few fossils 
were then found in it, and the knowledge then possessed of geology and 
palzontology would perhaps not have sufficed to settle this question 
even if the collections had been richer. Since the suspension of the for- 
mer survey the Waverly group has been a fruitful subject of discussion 
among geologists; and there is perhaps no one of our formations about 
which there has been so much difference of opinion. From the remote- 
ness of the localities where the rocks which were compared with the Wa- 
verly group were exposed, and the marked differences exhibited by their 
fossils, it was Impossible to apply to the question of equivalence an ex- 
perimentum crucis by which it could be definitely settled. The space 
which separated the fields of observation of the eastern and western 
