300 AUGUST F. FOERSTE 



at distinct angles with the inclosing rock. Moreover, the upper 

 surface of the larger pebbles rarely projects distinctly above the 

 surface of the inclosing rock, but more commonly is about at the 

 same level as the latter. On careless examination, the pebbles 

 appear merely as adhering remnants of the next overlying layer of 

 rock. They are distinguished chiefly by the finer grain of the peb- 

 bles, frequently accompanied by a difference in color and by a 

 difference in the character and location of the stratification planes. 



There is no evidence that the larger pebbles, a foot or more in 

 diameter, ever were turned over so as to present the lower instead 

 of the upper surface of the original rock stratum. Perhaps this 

 statement could be made with equal accuracy of any pebble 6 inches 

 in width. The largest pebbles, so far found, which give evidence 

 of having been overturned before being imbedded have a width of 

 almost 3 inches, although the length may equal 6 inches. In one 

 of these pebbles, obtained from layer D in Elk Run, a thin growth 

 of Ceramoporella ohioensis on one edge of the pebble overlaps both 

 the upper and the lower surface of the latter by fully an inch, and 

 additional growths of the same species occur on the lower surface, 

 the entire width of the pebble being 2f inches. The incrusting 

 specimens of Protarea frequently occur on the upper surface of the 

 pebbles, often several colonies on the same pebble, and these colonies 

 frequently overlap the lateral edges, but never occur on the lower 

 surface of the pebbles. 



The size and the angularity of the larger pebbles suggest that 

 they have not been transported very far. The absence of over- 

 turning of these larger pebbles also suggests only a short distance of 

 transportation. The very irregular surface features of the very 

 fine-grained pebbles, among which evidences of overturning are 

 more frequent, suggest washed lumps of partly indurated calcareous 

 mud rather than strongly eroded and frequently overturned rock. 



Although rock similar to that forming the pebbles frequently 

 occurs immediately below the layer in the upper surface of which 

 the pebbles are imbedded, these ripple-marked layers frequently 

 are continuous over such large areas, as determined from exposures 

 along the lateral branches of streams, as to make the origin of the 

 pebbles from underlying strata more or less doubtful. Especially 



