512 GEOLOGY OF OHIO. 



Crab Apple — Pyius coronaria L. 



Honey Locust — Gleditschia triacanthos (rare) L. 



Pawpaw — Asimina triloba Dnnal. 



Bass-wood — Tilia Americana ." L. 



VI. GEOLOGICAI. STBUCTDBE. 



The geology of Darke county is preeminently 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 takes its name. 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 fact 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, it 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 exteiftsively 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 n» subsequent up- 

 heaval. A section of ten or twelve feet can be observed at the quarries, 



