888 GEOLOGY OF OHIO. 
,and follow them, and practical men in search of ore and coal, have long 
since learned their value. The reasons for their selection are obvious. 
Limestones are more individualized than any other strata with which 
they are associated. They differ from each other in structure, in color, 
in the presence or absence of fossils, and to some extent in the kinds of 
fossils when any are present. They are generally slow to decay, and 
thus their outcrops are marked in all natural and artificial sections. 
When they chance to weather easily, they give rise to soils as character- 
istic and as easily recognized as the limestones themselves. 
A. LIMESTONES OF THE HANGING ROCK DISTRICT. 
There are six principal limestone horizons in the Hanging Rock Dis- 
trict. Five of them stretch through the whole field, and furnish the 
means of establishing the stratigraphical order of every portion of it. 
They are named as follows, being numbered in ascending order, 
6. Ames or Crinoidal Limestone, 
Cambridge or (locally) Black Limestone. 
Shawnee or Buff Limestone. 
Hanging Rock or Gray Limestone. 
Zoar or Blue Limestone. 
Maxville or (locally) White Limestone. 
b= OG E> x 
The lowermost stratum or the Maxville, is very much less extended 
than the rest. It is shown indeed only in isolated patches, and much 
remains to be learned of its development in the district. While it can- 
not be claimed as a conspicuous or steady horizon, yet in such portions 
of the field as it occupies, it serves a very useful purpose in establishing 
the true order and system of the Coal Measures. | 
These several limestone horizons are separated from each other by ap- 
proximately equal intervals, which vary, however, in different parts of 
the field, being generally increased as they are followed southward. In 
the Hocking Valley, the intervals are about one hundred feet. Thus 
the distance from the Maxville Limestone to the Zoar, ranges from one hun- 
dred toone hundred and thirty-five feet. No betteraveragecan be given for 
the interval between the Blue Limestone and the Gray (Hanging Rock) 
Limestones than one hundred feet. Again, the Shawnee or Buff Lime- 
stone lies one hundred toone hundred and ten feet above the Gray. The 
Cambridge is about one hundred feet above the Shawnee and, finally, the 
Ames ranges between eighty-five and one hundred and twenty feet above 
the Cambridge. In southern Vinton county, the interval between the 
Maxville and the Blue Limestone is a little less than that already given 
—viz., ninety feet. From the Blue Limestone to the Gray, the distance 
has been increased to. one hundred and twenty orone hundred and forty feet, 
