512 GEOLOGY OF OHIO. 
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Bawpaw—-Asimina trilobaye i. fle. spake ca say ale Ga rele reco ue Se Dunal. 
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VI. GEOLOGICAL STRUCTURE. 
The geology of Darke county is preéminently the geology of the Drift, 
but one rock-formation being exposed within its entire borders. This 
formation belongs to the Upper Series of the Niagara Group, known as 
the Guelph or Cedarville beds, and is very fully described by Professor 
Orton in his reports on Clarke and Greene counties. It is supposed to be 
identical with the Leclaire of Iowa, the Racine of Wisconsin, and the 
Guelph of Canada, from which it takesitsname. Although there are but 
five exposures, there is no doubt but that these beds compose the entire 
rock surface. Knowing positively, as we do, of the east, middle, and 
south-west, from the outcrop of the limestone itself, the other portions of 
the county are rendered almost equally certain from the facet that the ad- 
joining parts of all the counties, north, east, and south, present precisely 
the same phase—the same being exposed at Celina and Fort Recovery, 
Mercer county, at Covington, Miami county, and New Paris, Preble 
county. It was formerly thought by some members of the Survey, that 
the Waterlime Group extended into the northern part of the county. 
This might have been highly probable before the Glacial Epoch, but, 
being evidently of no great thickness, if must have been removed during 
that period of erosion. | 
As remarked before, in speaking of the Surface Features, little can be 
known of the effect of its contour upon the topography of the surface. 
One instance, however, was given of an eroded basin immediately be- 
neath the present location of Greenville. The strata where revealed, 
with but one exception, appear quite horizontal. 
The Guelph rocks are most extensively laid bare along Greenville 
Creek, and at Bierley’s, Hershey’s, and Roesser’s quarries in south-west 
quarter, section twenty-seven, Adams township. They form the bed 
of the creek here for a quarter of a mile or more. The quarries are situ- 
ated in the bottom of the valley or ravine, and are covered with about 
two feet of dark red clay or loam, mingled with the decomposed lime- 
rock, and strewn with heaps of large drift bowlders. The banks are 
_ twenty or thirty feet in height, and composed of yellow clay and hard- 
pan. The beds of limestone here appear perfectly horizontal, having 
been deposited (as indicated by the character of the rock) in a quiet and 
shallow sea, and having witnessed little disturbance and no subsequent up- 
heayal. A section of ten or twelve feet can be observed at the quarries, 
