THE CARBONIFEROUS SYSTEM. 171 
since it underlies, generally immediately, the Putnam Hill limestone. 
Coal No. 6 is equally unmistakable, as it is the thickest and most contin- 
uous of all the lower group of coals. In the valley of the Killbuck, above 
Millersburgh, Coals No. 4 and No. 6 are not more than 25 feet apart, but 
in tracing them down the Killbuck to the Tuscarawas this space con- 
stantly increases until it reaches a maximum of 100 feet. In the valley 
of the Tuscarawas I have traced these coals from Zanesville to near Mas- 
sillon. Along this line of observation the variation in the distance 
between them does not exceed 25 or 30 feet, as it follows nearly the center 
of the local basin to which reference has been made on a preceding page. 
But on either side of this basin the interval diminishes to one-fourth of 
its maximum. 
The intervals between Coals No. 6 and No. 7 I have found to vary, at 
localities examined by myself, from 54 to 100 feet, diminishing toward 
the east. 
The distance between Coals No. 6 and No. 8 has been shown by our 
measurements to vary in Jefferson county alone from 498 to 564 feet. 
Along the western line of outcrop of these coals the interval is from 400 
to 430 feet. 
The distance between the Pittsburgh coal and the Crinoidal, or Ames, 
limestone is, in central Ohio, from 140 to 150 feet, while in Jefferson 
county it is 225 feet. The two last-mentioned members of the series are 
so continuous and so strongly marked by individual characters that there 
ean be no mistake about their identification ; and it is also true—here as 
in the lower intervals—that the increase or diminution is found to be 
progressive, according as the line of observation is carried in one or the 
other direction. This could be shown by reporting the intervals at vari- 
ous points intermediate between the maxima and minima which I have 
given, but this is scarcely necessary here, since many of these facts have 
been given on the preceding pages. 
The remarkable variation in the interval between Coals No. 8 and No. 
10, described in the report of Prof. Stevenson on Belmont county, has 
been alluded to in the sketch given of the Upper Coal Measures. The 
accuracy of the observations reported by Prof. S. has been called in ques- 
tion, but justice to him requires that I should state that they have been 
fully confirmed by tracing the coals which overlie the Pittsburgh seam at 
Bellaire north and west to the points of their successive disappearance ; 
while the identity of Coals No. 8 and No. 10, in their outcrops in eastern 
and western Belmont county, has been demonstrated by following them 
around through Guernsey, Harrison, and Jefferson counties, from one 
locality to the other. It has thus been proved that between Barnesville 
