Position and thickness of the Strata. 



177 



thick ; and it may be much more. The stratum of sand above the 

 clay is so much mixed in its upper part with diluvium, that I can 

 form but a very vague idea of its thickness. It can, however, hardly 

 be less than 15 or 20 feet. 



I have no evidence that a stratum of sand is found beneath the 

 clay in the Connecticut valley ; though, as already mentioned, there 

 are sometimes numerous alternations of the sand and gravel in nar- 

 row strata, near the line of junction. Mr. Smith also states, that in 

 excavating the canal at Enfield Falls, the clay beds were entirely 

 cut through ; and that between the beds and the new red sandstone, 

 was found a confused mixture of rounded masses of sandstone, green- 

 stone, granite, and other primitive rocks, mixed with red clay from 4 

 ta20 feet thick. This is an interesting fact: because it proves that 

 in the interval between the deposition of the sandstone and the terti- 

 ary formation under consideration, fmviatile and diluvial currents, 

 similar to those that have since existed, must have been in operation. 



The surface of the tertiary formation in the vicinity of Boston has 

 been so much acted upon by diluvial currents, that as already re- 

 marked, I have been at a loss whether to describe it as tertiary or 

 diluvial. Eut there is no doubt, I believe, that genuine clay beds, or 

 layers of clay, do exist not far beneath the surface. This clay is 

 represented by the well diggers as extremely hard : and underneath 

 it, are layers of sand and gravel. It is from 70 to 120 feet thick; 

 and when perforated, water rushes upwards with great violence. 

 The only genus of organic remains found in the tertiary of the Con- 

 necticut valley, I have discovered also in the clay at Charlestown ; 

 unless I have greatly misapprehended its characters. But the same 

 genus occurs alsoin the clay beds of Nantucket; which I have been 

 inclined to consider as belonging to the Plastic Clay : so that this 

 relic does not seem to afford much aid in determining the relative 

 antiquity of these several beds. 



As to other limited patches of tertiary in the State, (excepting the 

 Plastic Clay, which occupies its southeastern part,) I know almost 

 nothing. I have only seen pits opened in them occasionally, for the 

 purpose of making brick. But although the clay appears to be min- 

 eralogically identical with that in the Connecticut valley, yet I have 

 met in it with no sort of organic relic. These beds must vary in rel- 

 ative level several hundred feet; occupying as they do, the depress- 

 ions of the surface in the various formations. This fact precludes 

 the idea of their having been deposited by the same body of waters,, 

 23 



