Plastic Clay. 



201 



any variegated clays occur above the green sand at Gay Head : but a 

 blue clay only. And probably by penetrating deeper at Duxbury, the 

 variegated clays might be brought to light. A question hence arises 

 whether the green sand ought to be regarded as belonging to the plas- 

 tic clay? Some writers do, indeed, limit the plastic clay formation 

 to the variegated clays : but that the lignites and green sand belong 

 to the same formation, (understanding by that term, a consecutive and 

 parallel series of strata produced during the same geological period,) 

 there can be no doubt. Not improbably, however, the green sand in 

 this country occupies the same situation with regard to the variega- 

 ted clays, as the London clay does in England with respect to those 

 clays. But probably the London clay ought not to be separated from 

 the plastic clay. 



The facts discovered at Duxbury render it extremely probable that 

 the extensive sandy deposites in Plymouth and Barnstable counties 

 are underlaid by plastic clays ; although the powerful diluvial action 

 to which that region has been subjected, may have spread these sands 

 over other formations. The sands that occur along the west side of 

 Narraganset Bay, in Rhode Island, in the vicinity of Providence, 

 may probably belong to the same formation. It is to be hoped that 

 when more of the cliffs and pits in this formation have been exam- 

 ined, a richer variety of organic remains will be brought to light. 



Relative Position of Plastic Clay. 



In Europe plastic clay lies immediately above the chalk. But in 

 this country the chalk appears to be wanting ; and Dr. Morton has 

 shown satisfactorily that its equivalent is the Ferruginous Sand For- 

 mation.* In the middle and southern States, the plastic clay lies im- 

 mediately above this ferruginous sand. But I have no evidence that 

 this is, or is not the case, in Massachusetts, except analogy ; which, 

 in the present case, must be considered very strong in favour of the 

 affirmative. The spot most likely to afford light on this point is Gay 

 Head. But the action of the sea has been so powerful there, that 

 little of the ferruginous sand, if it once existed, has been spared. 



Extent of the Plastic Clay Formation. 



The patches of this formation that have been described in Massa- 

 chusetts are doubtless only the remnants of a vast extent of these 



* Journal Acad. Nat. Sci. Vol. 6 : also Am. Journal of Science Vols. 17 and 18. 



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