94 PECULIAR STPwUCTURE OF CLARK'S CLAY-BEDS. 



circular eddies abated, erosion ceased, and as they became 

 still more gentle, clay was deposited, its layers becoming 

 more and more horizontal with the constant lessening of 

 the force of the circular currents. 



The difficult problem, however, in this case, is to ac- 

 count for the phenomena presented immediately outside 

 of the clay deposits ; the sudden termination of the 

 containing beds of loose stratified sand in a vertical 

 plane, the occurrence of great beds of coarse, pebbly 

 drift immediately beyond the sand and bounded on the 

 inside by the same vertical plane ; also, the presence of 

 a wall of solidified material between these two. I can 

 find no report of any similar phenomena. 



Any theory seeking to explain the formation of this 

 vertical wall should account for the following facts : 



1. The existence of such a solid wall, either single or 

 double, in a vertical position between two high mounds 

 of loose shifting material. 



2. The fact of the different character of these materials 

 inside and outside of the walls respectively. 



3. The entire want of conformity between the inner lay- 

 ers of fine sand and the wall, giving the impression that 

 the method of deposition of the wall had resulted in 

 cutting them down vertically ; and, in striking contrast, 

 the evident disposition of the outer layers of stratified 

 gravel, (as seen in figs. 3 and 4), to conform to the verti- 

 cality of the wall as they approach it. 



4. The fact that the inner surface of the wall is com- 

 paratively smooth, while the outside is very rough and 

 irregular. 



5. The fact that at certain limited areas the coarse, 

 pebbly drift of the outside actually traverses the wall 

 and appears inside of it, and as a component in its own 

 structure. 



6. The fact that the wall in some parts follows the out- 

 line of the upper edge of the single conical clay-bed 



78 



