666 GEOLOGY OF OHIO. 
springs that issue from this horizon wherever the drainage allows. It 
has already been remarked that the lower beds of the Clinton are sandy 
in texture. At many points they are extremely friable, and are, conse- 
quently, very easily removed by the underground streams that are flow- ; 
ing at this level, and, as a consequence, small caves frequently occur at 
the base of the series. In other cases sink-holes are found, which are 
due to the same general cause. By the solution of the rocks along the 
lines of the divisional planes or joints that traverse them, free way is 
opened from the surface to the water-bearing shales of the Cincinnati ~ 
group, and streams of small volume sometimes drop suddenly to this 
horizon to emerge again along the outcrops of the formation, perhaps at 
a distance of miles even from the point of descent. One of the best 
known of these sink-holes is found very near the intersection of the 
Xenia and Fairfield Pike with the Dayton and Yellow Springs Pike. 
The stream that here drops from the light of day to these subterranean 
recesses comes out again a mile or more to the southward, re-enforced, 
doubtless, by others that have shared a like fate, as the head spring of 
Ludlow Creek—one of the finest fountains of the county. These sink- 
holes have been sometimes deserted by the water-courses that have 
helped to fashion them, in which cases they have frequently been con- 
strued, in the neighborhoods in which they occur, as abandoned “lead 
mines.” Some portions of the county are full of circumstantial tradi- 
tions of lead veins being worked by the Indians here. It is scarcely 
necessary to say that the civilized occupants of Greene county know a 
vast deal more of its geological structure and mineral resources than any 
of their uncivilized predecessors have done. There is not a shadow of 
reason for believing in the existence of metallic veins of any sort within 
its area. 
The limestone terminates at its upper limit variously. The most 
characteristic mode is in a foot or two of very fine-grained, light-blue 
clay or marlite. This, it will be remembered, is the usual mode in Mont- 
gomery county, where the horizon is found to be one of great paleon- 
tological interest. In Greene county, however, when the marlite occurs 
it is sometimes destitute of fossils. It can be seen at the base of McDon- 
ald’s quarry, south of Xenia, and at a few points along the Grinnell 
pike, near Yellow Springs. | 
When the blue clay is not shown there is no change in the composi- 
tion of the limestone for its uppermost ten or fifteen feet, but there is 
always a very marked transition in passing to the lowest beds of the 
_Niagara group. : 
The uses of the Clinton limestone are much less important now than 
