INTRAFORMATIONAL PEBBLES IN RICHMOND GROUP 293 



The line-grained mud of which these pebbles are composed 

 incloses but few fossils. In three pebbles Lophospira bowdeni 

 was found, and one pebble contained a ventral valve of Dinorthis 

 subquadrata, a characteristic fossil of the Liberty formation, 

 unknown in the underlying Waynesville strata. In other words, 

 there is no reason for believing that the rock of which the pebbles 

 are composed is older than that of the formation in which the peb- 

 bles now occur. In fact, at several localities along the creek, the 

 rock immediately underlying the pebble-bearing layer is sufficiently 

 similar to the rock forming the pebbles to have given origin to the 

 latter. 



These fine-grained pebbles usually do not exceed 6 inches in 

 length, 3 inches in width, and an inch in thickness, but specimens 

 12 inches long, 7 inches wide, and 2 inches thick are known, and 

 one pebble 18 inches long, n inches wide, and 3 inches thick was 

 observed. 



2. The second type of pebbles usually consists of a fine-grained 

 blue limestone, in which the grain is distinctly less fine than in the 

 first type. The granular structure usually may be recognized with- 

 out the assistance of a magnifier. Worm-burrows usually are 

 absent, and no incrusting bryozoans or specimens of Protarea have 

 been observed. The color of the rock is bluish gray, similar to that 

 of the inclosing rock, and the outlines of the pebbles may be dis- 

 tinguished from the latter chiefly by the finer grain of the rock 

 forming the pebbles, and usually also by differences in the strati- 

 fication planes running through the rock. Such fossils as occur 

 in these pebbles suggest the Liberty age of the rock from which the 

 latter were derived, and the source of this rock could have been one 

 of the underlying layers within this formation. 



The pebbles of this second type usually are relatively thin and 

 flat. The upper and lower surfaces usually are parallel, the lateral 

 margins often being vertical or rounding only moderately into the 

 upper and lower surfaces. In vertical cross-sections, therefore, 

 the pebbles appear angular at the margins. Angularity fre- 

 quently characterizes also the lateral outlines, as observed from 

 above. In other words, the pebbles frequently appear broken off 

 from thin layers of limestone, without much rounding. Some of 



