296 GEOLOGY OF OHIO. 
This composition, if maintained, will entitle the coal to a leading 
place, not only in this field, but in the State at large. There are few 
mines in Ohio that yield better results than those given above. 
Some irregularity was found in driving northward from the shaft, 
the jtrouble being due, as usual, to a horseback of sandstone, which 
reduced the coal. The direction of the sandstone trough, as far as could 
be judged at the time this record was made, seemed to be northwest. 
It is claimed that drillings have established the existence of the seam 
in good condition to the northward of this property. 
A peculiarity of this mine is the partial replacement or conversion 
of the draw slate into an impure coal. In some cases, layers of coal 
one or two inches thick come into the slate, and in others nearly a foot 
of unmarketable coal is added to the seam. 
The {pillars of the mine have been so far left 25 feet wide. The 
bottom is harder than in the northern mines, but if pillars of this size 
can maintain the entries, very different conditions will be shown here 
from those that obtain in the remainder of the field. ‘The structure of 
the Hartford coal is shown in the accompanying plate: 
FLGURE LXX 
STRUCTURE. OF CAMBRIDGE GOAL. SEAM 
AT SPAIDS MINE HARTFORD. | 
Root Shales conbaiaing 
coal | 
steaks 
SU rere Seung oe ee eter 
Slate SSNS oe 
Goal ‘Lb 
ool “Lower Reuclku____ 
As to the limits of the field on the south and west, it is not possible 
yet to speak with assurance. Exploration has not, by any means, been 
carried far enough to assign boundaries. Experience from similar fields 
justifies the belief that many additions will be made to the first found 
areas. 
