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 

 treason for believing in the existence of metallic veins of any sort within 

 ats 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 palseon- 



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- 

 tiqn 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 usesof the Clinton limestone are much less important now than 



