22 



The only rocks in Massachusetts that have been observed 

 passing below the Huronian system, or cutting through its lower 

 members, are those composing the Naugus Head series ; and 

 this, together with its crystalline character and immense dis- 

 turbance, convinces me that this series is the oldest in the State. 

 In the light of our present knowledge the conclusion cannot be 

 avoided, that the Naugus Head series is the real base of the 

 geological column of Massachusetts. 



The coarse-grained, readily disintegrating exotic diabase, so 

 extensively quarried in Medford, — and also occurring in Somer- 

 ville, Brookline, and, probably, other places, — bears a strong 

 resemblance to certain members of the Naugus Head series, es- 

 pecially to the coarse pyroxenic rock of Nahant and Salem 

 Neck ; and the idea is natural that they are extruded portions 

 of this series, which may, I think, be regarded as the probable 

 seat of many of the eruptive masses cutting the newer formations 

 of this region. 



For the accurate identification of these exotics in Medford, 

 Somerville, Brighton, Brookline, etc., we are indebted to Mr. 

 M. E. Wadsworth. 1 This observer has found that the principal 

 dark constituent of these rocks, in the unaltered condition, is 

 always pyroxene, never hornblende ; a fact which accords well 

 with the general conclusion already stated with respect to the 

 composition of the basic rocks of the Naugus Head series. The 

 forms and general relations of these masses will be more fully 

 described in connection with the uncrystallines which they 

 intersect. 



The small area of basic exotics exposed near the head of 

 Washington avenue, in Chelsea, has been doubtfully referred to 

 the Naugus Head series. North of Wenuchus Lake, in Lynn, 

 is a hill composed of a nearly pure feldspar rock, — a 

 coarse, whitish fieldspar, apparently the same as that so well 

 developed on the Beverly shore. The rock is massive, having 

 the aspect of an exotic ; and around the base of the hill it can 



i Proc. B. S. N. H., xix., 217. 



