WILLIAM B. DWIGHT. 93 



searching, I succeeded in tracing the remains of the wall 

 sufficiently to confirm his statement. The thickness of 

 the wall varies from one inch to two feet, the latter width 

 generally occuring only where the wall is double. 



A very peculiar feature in the case is the fact that 

 almost everywhere the layers of very fine argillaceous 

 sand (sometimes alternating with layers of sandy clay) 

 which forms the enclosing envelope of the main clay- 

 beds (p. tigs. 1, 2 and 3) were lying inside of the vertical 

 wall, while against the outside of it was everywhere a 

 mass of coarse sand gravel and pebbles, in stratified 

 deposits. This wall was everywhere, therefore, a dis- 

 tinct boundary between these two classes of deposits. 

 It should also be noted that as the wall was vertical, 

 while the exterior layers of the clay-beds sloped inwards, 

 the layers of fine sand within were of very little extent 

 near the top, but widened out rapidly near the bottom. 

 At the level of the working-floor there was more than 

 five or six feet of sand. The outside drift was in a 

 series of mounds. The mound lying against this partic- 

 ular clay-bed was about forty feet high, or about half 

 the height of the mass of clay. The mutual relations of 

 these various portions was finely shown in the walls of 

 the one hundred and twenty foot passage-way, which 

 cut a vertical section through the whole, as shown in 

 figs. 3 and 4. Fig. 1 represents a vertical section through 

 the mass of clay in a northerly and southerly direction. 

 Fig. 2 shows the position of the vertical wall with refer- 

 ence to the three clay-beds. 



With regard to the particular form in which the clay 

 was deposited, the facts seem to me to show that in this 

 case there was originally (1) an extensive and deep de- 

 posit of fine argillaceous sand ; (2) a series of conical 

 excavations in this sand-bank caused by whirlpools, 

 themselves occasioned by some changes in the shape of 

 the adjoining river-banks ; (3) as the violence of these 



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