Ripley Island Area of Southern Indiana. 327 



and most hardened, and those transported the shortest distance, 

 were left most angular. Occasionally fragments of Ordovician 

 limestone, formed by the detrital remains of shells and bryo- 

 zoans comminuted so as scarcely to be recognizable, have been 

 caught up by the Clinton. In these cases the fragments of 

 limestone consist usually of thin slabs. It appears that this 

 limestone of detrital origin solidified more rapidly than the 

 very fine-grained mud forming the white limestone, probably 

 because the detrital limestone permitted a freer circulation of 

 water. In these detrital limestone pebbles Rafinesquina 

 alternate, and other Ordovician fossils are found. The local- 

 ities from which pebbles in the Clinton have been recorded 

 are indicated on the map by the letter P ; they undoubtedly 

 include only a small part of the localities at which pebbles 

 occur. 



South of Laurel, those exposures of the Clinton which indi- 

 cate a detrital origin usually have a salmon-brown color. The 

 detrital material usually is fairly coarse. The absence of the 

 finer material which must have resulted from the attrition is 

 probable due to currents which were strong enough to sweep 

 along the finer material but left the coarser particles behind to 

 form the Clinton deposit. Sometimes this fine-grained material 

 appears to have been allowed to deposit for a short time, pro- 

 ducing thin layers, but as a rule these layers were swept away 

 again later; however their former existence is suggested by the 

 presence in the salmon-brown Clinton of thin lenses of fine- 

 grained whitish limestone, usually widely separated but some- 

 times connected by very attenuated sheets of the same material. 

 Some of the localities at which these white lenses occur in the 

 salmon-brown Clinton are indicated on the map by the letter 

 L. The lack of distinct bedding or the presence of cross bed- 

 ding is another evidence of the deposition of the Clinton under 

 the influence of rather strong, irregular currents. 



Where the Clinton is absent in Indiana, the lowest layers 

 of the Osgood rest directly upon the Madison bed. At one 

 locality 4 miles west-southwest from Osgood, a single pebble of 

 salmon- brown Clinton rock with blotches of black was found 

 enclosed in the white limestone forming the base of the Osgood 

 beds, 2 inches above the Ordovician. The pebble was 3 inches 

 long, almost 3 inches wide, and half an inch thick. This is 

 the only pebble found at any point in the Osgood bed. Imme- 

 diately beneath was clay with Tetradhtm, and farther down 

 occurred limestone including concretionary masses or some 

 species of Strephochetus. 



The presence of areas from which the Clinton is absent, 

 surrounded by areas in which the Clinton contains pebbles of 

 Ordovician origin, is an indication of course of unconformity 



