HURON COUNTY. | 307 
and Ashtabula counties, where borings for gas wells have given accurate 
measurements of the Erie shale, would be as follows: 
Prorine SECTION SHOWING RELATIONS OF CLEVELAND, ERIE, AND HURON SHALES IN 
NORTHERN OHIO. 
A—Cleveland shale. 
GuliE eo atates 
The included Hrie 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 ereat 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 canne! coal, containing in places. the remains of 
plants acccmpanied with thin films of true coal. They also frequently 
include thin strata of blue argillacecus 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. Fissuresin 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 aium is sometimes 
seen three-fourths of an inch in thickness; and occasionally a nearly 
pure sulphur of equal thickness may be observed. 
