GEOLOGICAL SCALE AND STRUCTURE. 9 



pass with the driller as black shale. The Hudson shales are, thin in this . 

 northwestern corner of the state, the entire measure running as low as 

 three hundred feet, or even less. It seems probably, in view of this and 

 other facts, that they were lain down in a sea that grew shallow in that 

 direction, viz , the west. At least, there was a perceptibly smaller amount 

 o. 1 the fine sediments of which the formation consists brought in as we fol- 

 low it in this direction. To the eastward the greenish blue shales already 

 named are always found, and the series also thickens considerably in this 

 directiou. The Hudson river shales are everywhere fossiliferous, as the 

 fragments of corals and shells brought up in the drillings abundant 

 testify. Some of the fossils are identifiable in the fragments brought to 

 the surface by the driller. 



The Hudson River group occupies in its outcrop, about four thousand 

 square miles in southwestern Ohio, but it is doubtless coextensive with 

 the limits of the state, though under very deep cover in large part. The 

 shales of the series in in southwestern Ohio contain large quantities of 

 phosphates and alkalies, and the soils to which they give rise are pro- 

 verbial for their fertility. 



The presence of these fined-grained and impervious shales in so many 

 separate layers forbids the free descent of water through the formation. 

 In its outcrops, consequently, the Hunson River shales have no water sup- 

 ply, and, as found by the driller, they are almost universally dry. The 

 shales give rise to frequent "blowers" or short-lived accumulations of 

 high-pressure gas when struck by the drill, as is found in the experience 

 of many towns of western Ohio within the last few years, and it also 

 yields considerable amounts of low-pressure shale gas at many points in 

 the statt, some of which have proved fairly durable. 



4. The Medina Shale. 



A stratum of non fossiliferous shale, generally red or yellow in color, 

 and having a thickness of ten to forty feet, directly overlies the upper- 

 most beds of the Hudson River group at many points of outcrop in 

 southwestern Ohio. From its stratigraphical position, these beds were 

 referred to the Medina age in the reports of the Geological Survey for 

 1869, but the identification was considered as grobable, rather than cer- 

 tain. The occurrence of fifty to one hundred and -fifty feet of red shale 

 in the deep borings that have recently been carried forward in northern 

 Ohio, at exactly the place in the general scale where the Medina forma- 

 tion should be looked for, and so much nearer to the outcrops of the 

 formation, that its continuity with these was hardly to be questioned, 

 this fact, taken in connection with the occurrence of like beds of red shale 

 holding the same relative position in all the deep borings in the central 

 portions of the state, gives full warrant for counting the Medina epoch 

 duly represented in the outcropping strata of southwestern Ohio, accord- 

 ing to the determination above named. It occurs here only in included sec- 

 tions, its thin and easily eroded beds never being found as surface 



