THE OIL- AND GAS-PRODUCING ROCKS OF OHIO 837 



sand is made more certain by the fact that the Cambridge lime- 

 stone lies immediately below or is separated from the sand by a 

 thin bed of shales. Unfortunately these limestones are not 

 recognized in Monroe and the eastern part of Washington 

 counties, and hence the sand in question often cannot be identi- 

 fied there with certainty. In such localities any shallow sand, 

 especially if it makes a show of oil or gas, is known as the Cow 

 Run. The relation of the sand to the two limestones places it 

 about 100 feet above the base of the Conemaugh formation. 



If the Berea grit be taken as the type of a persistent stratum 

 of sandstone, the First Cow Run sand may properly be selected 

 as the representative of the opposite type. It is the most patchy 

 of the oil or gas rocks of Ohio. The maximum thickness of 

 the formation may be taken at 50 feet, but even a short distance 

 from this the stratum may become very thin or actually dis- 

 appear entirely. The texture varies much, and where product- 

 ive is sometimes conglomeritic. Pebbles three-fourths of an inch 

 in diameter have been found, and those one-fourth of an inch 

 are common. The pebbles consist of quartz, but small grains of 

 other minerals are found. From this type the formation changes 

 to a hard compact sandstone. 



The sand is an important source of oil in Morgan and Wash- 

 ington counties. It seems to have been first struck at Macks- 

 burg in i860, where it is known as the 140-foot sand ; this being 

 the depth at which the sand was found in the valley at that 

 place. In 1861 the sand was found at Cow Run and has there 

 been a producer ever since. It is seen that the latter name has 

 not the claim of priority, but it is so much more widely used 

 that it is here retained. 



The Mitchell sand. — This is the source of a small oil field a 

 few miles east of Marietta. The sand is comparable in thick- 

 ness with the First Cow Run, but in texture is less conglomer- 

 itic. As has been shown, it lies about 90 feet below the Pitts- 

 burg coal, or 200 feet above the First Cow Run sand. Its place 

 is near the summit of the Conemaugh formation. Wells in this 

 sand commonly begin with a relatively large production, but 

 decrease very rapidly. 



