326 Foerste — Ordovician-Silurian Contact in the 



tinues to diminish, but southwestward, in the western part of 

 Ripley and the adjacent parts of Jennings and Decatur counties, 

 the Clinton is entirely absent. These facts were entirely 

 unexpected. In keeping with preconceived views of the early, 

 pre-Silurian origin of the Cincinnati geanticline, it was believed 

 that after crossing the axis of the geanticline the thickness of 

 the Clinton would increase instead of decrease westward. 

 Instead, a fairly regular decrease in thickness across the axis of 

 the geanticline was noted, beginning with the most north- 

 eastern exposures in Ohio, and continuing to the most south- 

 western of the series of exposures so far discussed. 



The area, or areas, from which the Clinton is absent is here 

 called the Ripley island or islands. The outlines of this area 

 are known only along the eastern border, and even here much 

 remains to be determined. The facts so far discovered ' are 

 indicated on the accompanying map (Plate XVII),. East and 

 southeast of the Ripley island area the Clinton contains pebbles. 



These pebbles which occur in the Clinton are usually confined 

 to the lower 4 to 6 inches of the formation. They are com- 

 monly not more than an inch in length, although pebbles 2 and 

 even 3 inches in length are recorded from several localities, and 

 at one locality thin flat pieces of rock, 6 to 8 inches in length, 

 were included. Most of these pebbles consist of a very fine- 

 grained white limestone, belonging to the very top of the 

 Ordovician of this part of Indiana. The most common fossil 

 in this white rock is Tetradium minus, but this rarely appears 

 in the pebbles. Not infrequently the white limestone occurs 

 directly beneath the Clinton ; occasionally in such situations it 

 is cracked and the cracks are filled with the salmon-brown 

 detrital material of which the Clinton is formed in this part of 

 Indiana; the contrast in color is striking. Occasionally the 

 white limestone is cracked also where it occurs at some distance 

 beneath the Clinton and the cracks are filled with clay. Some- 

 times worm-borings penetrate for several inches into the top of 

 the white limestone, and the cavities are filled with the salmon- 

 brown Clinton material. The pebbles formed from this lime- 

 stone are frequently very angular in shape, sometimes having 

 the appearance of derivation from some brecciated rock, at 

 other times with the corners conspicuously rounded. It appears 

 that before the deposition of the Clinton, this white limestone 

 was a soft calcareous mud into which worms could bore ; owing 

 to shrinkage it cracked into rather small irregular fragments ; this 

 shrinkage may have been caused by drying owing to exposure 

 of the surface to the sun ; the surface of the Clintonless area 

 may have been in the condition of flats within the reach of high 

 tides; the fragments produced by cracking were often washed 

 toward regions of deposition ; those longest exposed to the air 



