EfeUMlS, AND GENERAL DISCUSSION. 



533 



time of the formation of the larger portion of them. It would seem 

 from these observations that the calcareous mass was the result of the 

 action of thermal waters during the time of the dying out of the vol- 

 canic forces which have been shown to have formerly been active in 

 Eastern Massachusetts.* 



The limestone localities at Chelmsford, so fully described by Mr. 

 Burbauk, were also carefully examined by us. They are included in 

 gneiss, and are extremely irregular, sometimes coinciding in direction 

 with the lamination of the enclosing rock, and at other times cutting 

 across it. So irregularly are the gneiss and limestone locked together 

 that their relations to each other could, in our judgment, only be ex- 

 plained by admitting that the calcareous material is a segregated 

 deposit. This "eozoonal" limestone is entirely crystalline, and filled 

 with scapolite, actinolite, and other silicates ; while tlic Eozoon is most 

 abundant and best preserved in the most decidedly crystalline portions 

 of the calcareous deposit. No dike was seen to be cut by the limestone ; 

 but one, of melaphyr, about two inches wide, was seen traversing it. 



We have also studied the deposits of limestone in Newbury, known 

 as the "Devil's Den" and "Devil's Basin." The relations of the cal- 

 careous mass to the enclosing rock at these localities are very similar 

 to those already indicated for the others, and do not allow of explana- 

 tion in any other way than by admitting that the limestone is a segre- 

 gated deposit. The evidences of intense chemical action are here also 

 plainly revealed by the presence, in the calcareous mass, of silicates, 



Moreover, the rock in which the lime- 

 stone is enclosed — the country rock — is, as we think, an eruptive one, 

 belonging with the older basalts, but of course much changed from its 

 original character. 



This view of the segregated nature of these deposits was, moreover, 

 entirely freed from anything which it might have of a theoretical nature 

 by our finding at Devil's Basin — the locality in which the Eozoon 

 occurs — that the country rock was cut by dikes of eruptive rock, 

 which squarely abutted against the limestone on the side without 

 intersecting it. Also, the ends of the dikes next to the limestone 

 Bhowed the same intense chemical action and resulting secondary sili- 

 cates that the country rock in hko position does. All this indicates, in 

 the most unmistakable manner, that the country rock was formed, and 

 that dikes traversed it, before the limestone luas there. In short, the cal- 

 careous mass has been introduced since the formation of the enclosing 



* Ilarv. Univ. Bull., No. 22, pp. 359, 360. 



such as wollastonitc and garnet. 



i'l 



MP 



