WILLIAM 15. DWIGHT. 95 



under discussion, while on the river-side it abandons the 

 individual contour, to extend along the common front of 

 the other two clay-beds. 



I rind it very difficult to frame a theory which would 

 reasonably and sufficiently account for these very curious 

 facts. I will give two explanations which have occurred 

 to me, of which the second seems the preferable one. 



On the first theory, the vertical wall may have been 

 formed as follows, while there was only a great con- 

 tinuous sand-bed outside of the clay to the east. Car- 

 bonated waters, holding calcium carbonate in solution, 

 may have been furnished from either of these two 

 sources: 1. The overhanging bluffs of Trenton and 

 Calciferous limestones, immediately adjoining the clay- 

 beds on the inner or w r est side. These limestones to-day 

 furnish streams in the immediate vicinity which deposit 

 large masses of calcareous tufa. 2. The clay itself 

 being exceedingly calcareous, its upper layer might have 

 furnished calcium carbonate to the flowing waters. 

 These waters, thus passing over the top of the imper- 

 meable clay beds toward their outer (eastern) rim, 

 would then have dropped over into the loose open sand 

 below, and, percolating easily through it, in a vertical 

 plane, it might have gradually cemented the sand grains 

 in its course into a wall. This wall, following the outr 

 line of the beds, was circular at the northern and southern 

 portions ; but, as the three contiguous circular beds 

 were at that time probably united at the top, the wall 

 along their river front necessarily was formed in a 

 straight line. This is illustrated in fig. 4. Where the 

 descending sheet of carbonated waters found an obstruc- 

 tion, it split, forming a double wall. This must have 

 taken place during an elevation, when the wdiole mass 

 was above water. Subsequently, either by sinking of 

 the land, or the rise of the waters in floods, the great 

 mound of fine sand must have been re-stratified. That 



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