286 T. O. MABRY 



more than a few feet. The lowest stratum (c) has been suffi- 

 ciently described above. 



This bowlder stratum (b) consists of rock materials similar 

 to those of the directly underlying or adjacent Lafayette ; so 

 from a lithological standpoint two views of the origin of this 

 stratum are possible. Either it and (c) have been derived from 

 the same pre - Lafayette formation, or formations, and the 

 apparent unconformity between them is to be regarded as a 

 "pseudo-unconformity," as explained by McGee, or the two 

 are distinct formations, and the upper one has been derived 

 from the lower. 



The latter I regard as the more probable for the following 

 reasons : 



1. The extent to which the underlying sands have been 

 eroded, and the very abrupt change from cross laminated sands 

 (seemingly a local delta deposit) underneath, to a bowlder 

 stratum of the character described — these seem to indicate 

 extraordinary conditions of deposition for the bowlder stratum, 

 and an amount of erosion of the underlying formation, which 

 could not be accounted for by a mere local shifting of currents, 

 with no appreciable changes of level nor consequent interval of 

 erosion. 



Smaller clay pellets, it is true, occur quite frequently else- 

 where, in the body of the Lafayette, but never so large, as far 

 as I am aware, as those just described. 



2. The size of many of these bowlders, and their frequent 

 angularity (which may be due in part, however, to subsequent 

 atmospheric action) , as well as their composition and physical 

 texture, render it highly improbable that they have been trans- 

 ported by running water for any considerable distance.. 



The Lafayette proper is about 150 to 180 feet thick in this 

 vicinity, as shown by recent well borings ; and the Lignitic, the 

 immediately underlying deposit, comes to the surface only at a 

 distance of several miles to the eastward. A well recently 

 bored upon the university campus, after passing through a few 

 feet of surface loam penetrated the Lafayette formation, and 



