246 GEOLOGY OF OHIO. 



from bituminous impurities. Such are especially the compact coral masses, 

 which make a purely white lime. The genera Stromatopora, Coenostroma, 

 and Cyathophyllum are most common. Cystiphyllum can also be distin- 

 guished in the rock, as well as Favosites. There is also a coralline form which 

 shows no evident structure, but seems to consist of a series of concentric 

 waves on a rock surface, spreading over a diameter of a foot or two. At 

 Mr. Myers's quarry these corals are especially displayed, making the stone 

 very irregular both as to color and bedding. Although the thickness of 

 the bedsf is usually from two to four inches, they are quite loose and often 

 lenticular. Thick films and scales of black bituminous matter spread 

 through it, giving rise to various local designations for the different por- 

 tions of the quarry. In some places the bituminous matter is evenly dis- 

 seminated through the beds, making the whole appear darker, even to a 

 brown ; in others it is gathered into scales, films, and pockets, which, com- 

 bined with the occurrence of the different corals, produces a very uneven 

 and unattractive stone. 



The Drift. — This deposit in Crawford county received special attention 

 during the progress of the Survey. I presents the usual characters of 

 a glacier deposit. Its most common aspect is that of a gravelly clay, else- 

 where designated hard-pan. This clay not only embraces gravel stones, 

 but also bowlders of all sizes, and has an average thickness of thirty 

 to fifty feet. Along streams, and in all valleys of erosion even where 

 no streams now exist, the bowlders belonging in this hard-pan are made 

 to appear superficial by the removal by running water of the clayey 

 ingredients. Occasional exposures of the Drift in fresh sections reveal 

 not only a confused mingling of clay, stones, and bowlders, but also in 

 some places an oblique stratification and perfect assortment of gravel 

 and sand. Such stratified beds pertain, for the most part, to the upper 

 portion of the Drift, and specially prevail in the eastern or sandstone 

 district. They give rise to springs of ferriferous water, and furnish that 

 of a great many wells. In counties bordering the lake shore this hard- 

 pan is frequently overlain by a fine clay arranged in horizontal lamina- 

 tions, but that form of the Drift is not met with in Crawford county. At 

 the bottom of the hard-pan, and separating it from the bed-rock, there is 

 very often a water-bearing layer of gravel and sand. 



As has already been remarked under the head of Surface Features, the 

 Drift of Crawford county lies in ridges crossing the northern and central 

 portions of the county. These are believed to be due to the temporary 

 halting of the margin of the ice-field when it occupied successively those 

 positions. These ridges are all situated so near each other, and some- 

 times become so involved with each other, that they seem to pertain to 



