THE CARBONIFEROUS SYSTEM. 139 
Coat No. 4. 
Throughout the greater part of the belt of outcrop of the Lower Coal 
Measures in Ohio, at a distance varying from 20 to 90 feet above Coal 
No. 8, another coal, another limestone, and another ore bed are found, 
which are no less marked and constant features in the series than those 
just described. The resemblance between the two groups is so close that 
it is not always easy to discriminate between them, and this has led to 
some errors in our earlier reports. 
The great variation in the interval which separates them has also led to 
some difference of opinion, and has been considered by those who hold to 
the theory of the strict parallelism of coal seams as proof of a want of con- 
tinuity and identity in one or both groups. These strata have, however, 
been traced with great care through many counties by several members 
of the Corps who have had much experience as coal geologists, and they 
are all agreed in regard to the relations of these limestone groups to 
each other and to the associated strata. They also coincide with me in 
the opinion that the kind of parallelism here shown is an argument 
against the theory which has been referred to, rather than against the 
continuity and identity of each group throughout the region it traverses. 
Where the interval between the limestones is considerable, two and 
sometimes three coal seams are found between them. With the excep- 
tion of Coal No. 4, these are generally quite local in their extent, and 
rarely attain workable thickness. One of these may be seen in the 
northern part of Tuscarawas county, between Dover and Mineral Point. 
It occupies about the middle of the space between the limestones, has a 
maximum thickness of about three feet, and is of inferior quality. In 
the Report of Progress for 1870 this seam is described and enumerated as 
Coal No. 4, but subsequent observation proved that it was so local and 
unimportant that it was regarded as unworthy to be numbered as one of 
our series of lower coals. It has, therefore, been designated in our later 
reports as Coal 38a. 3 
In the valley of the Killbuck and that of the Tuscarawas the lime- 
stones over Coals No. 8 and No. 4 may be traced almost continuously for » 
nearly 100 miles, where their relations to each other and the associated 
rocks are so apparent that no one can mistake them. MHere the distance 
between the limestones is found to vary from twenty to ninety feet, af- 
fording an excellent illustration of the local subsidences which took 
place during the formation of our Coal Measures, and which have so fre- 
quently occasioned a want of parallelism between our coal seams. 
Like the lower limestone seam, Coal No. 4 is exceedingly variable, 
both as regards its quality and thickness. It is also prone to divide 
