HUEON COUNTY. 307 



and Ashtabula counties, where borings for gas wells have given accurate 

 measurements of the Erie shale, would be as follows : 



Pi.OFiLE Section Showing Eblations of Cleveland, Erie, and Hukon Shales in 



Northern Ohio. 



West. East. 



A — Cleveland shale. 

 B — Erie stale. 

 C — Hnron shales. 



The included Erie shales measure fully twelve hundred feet in Lake 

 and Ashtabula, and not over thirty-two feet in Huron county. A simi- 

 lar thickening up of the strata between the Huron shale and the Coal 

 Measures is shown as explorations are carried south-ward along the west- 

 ern margin of the coal fields, indicating a long continued and great sub- 

 sidence to the south and east of Huron county, after the deposit of the 

 Huron shales. This subsidence was so general and rapid as to prevent 

 the growth of vegetation, except fucoids, and to afford deep water in 

 which molluscous animals were abundant. This state of things con- 

 tinued until the ushering in of the true coal period. 



HURON SHALE. 



These are highly bituminous black shales having somewhat the ap- 

 pearance of impure cannel coal, containing in places the remains of 

 plants accompanied with thin films of true coal. They also frequently 

 include thin strata of blue argillaceous shales containing very little 

 bituminous matter. Spheroid, and in the lower part of the Huron 

 shales, elongated concretions are very abundant, varying in size from a half 

 inch to fifteen feet in diameter. The smaller ones are composed almost 

 entirely of pyrites, the larger one of impure carbonate of lime. These 

 latter ordinarily show vertical lines of fracture and sometimes well- 

 marked horizontal lines of stratification. Fissures in them are frequently 

 filled with crystals of sulphate of strontia or of lime. A nucleus is 

 ordinarily found at the center, sometime organic, but oftener mineral. 

 The shales are so highly charged with sulphur and potash that in ex- 

 posures protected from the rain an efflorescence of alum is sometimes 

 seen three-fourths of an inch in thickness ; and occasionally a nearly 

 pure sulphur of equal thickness may be observed. 



