10 GEOLOGY OF OHIO. 



formations for large areas. There is good reason to believe that the 

 Medina formation is coextensive with the limits of the state, except in 

 the regions from which it has already been removed by erosion. 



The red color of the shales is persistent, but there are some well 

 records in which this color does not appear. This is especialh- true in 

 Allen count}', and to the westward and northwestward from Lima. Blue 

 shales alternate with the red in the eastern sections. In the western they 

 replace the latter. Thin beds of sandstone are found in the Medina. 

 especialh T to the westward. Small pebbles occur in some of these beds. 



5. The Clinton Limestone. 



The .Clinton group of New York appears as a surface formation in 

 Ohio only in the area already named. It forms a fringe or margin of the 

 Cincinnati group through ten counties, rising above the soft and easily 

 eroded rocks of this series, and of the previously named Medina shale, in a 

 conspicuous terrace. It is everywhere a well-characterized limestone 

 stratum. The stone is highly crystalline in structure, and is susceptible 

 of a good polish. In some localities it is known as a marble. A consid- 

 erable part of it, and especially the upper beds, are almost wholly made 

 up of crinoidal fragments. In thickness it ranges between ten and fifty 

 feet. Its prevailing colors are white, pink, red, yellow, gray and blue. 

 At a few points it is replaced by the peculiar form of hematite ore that is 

 elsewhere so characteristic of the formation. The ore is generally too 

 lean and uncertain to possess economic value, but it was once worked for 

 a short time and in a small way in a furnace on Todd's Fork, near Wil- 

 mington, Clinton county. 



The limestone contains throughout most of its outcrops a . notable 

 quantity of indigenous petroleum, but the only valuable accumulations 

 of oil and gas that have been found in it thus far have been brought to 

 our knowledge since 1885. It is the source of the low pressure gas of 

 Fremont (upper vein), and also of the important supplies at Lancaster, 

 Newark and Hadley Junction. In a few instances it has proved itself an 

 oil rock. Wells drilled to this horizon have in a few instances yielded 

 twent)' to thirty barrelsaday.thesupplybeingcontinuedfor several months. 



Under her-vy cover, and particularly in the new gas fields named 

 above, beds of sharp sandstone are sometimes interstratified with the 

 limestones. The main reservoir of the Lancaster gas is in fact a sandstone. 



In outcrop the stratum is porous, as a rule, and the water that falls 

 upon its uncovered portions sinks rapidly through them to the under- 

 lying shale (Medina), by which it is turned out in a well-marked line of 

 strong springs. 



In composition the limestone in its outcrops in southern Ohio is 

 fairly constant. All its most characteristic portions contain eighty to 

 eighty-five per cent, of carbonate of lime, and ten to fifteen per cent, of 

 carbonate of magnesia. At a few points, however, and notably at 

 Brown's quarries near New Carlisle, Clarke county, it appears as the 



