261 



With the exceptions just noted, the large area of smooth and, 

 for the most part, lowland, lying principally in Everett, Chelsea, 

 and Revere, affords but a single clue to the nature of the under- 

 lying rocks ; and this is the outcrop of rather nondescript ex- 

 otics at the head of Washington Avenue in Chelsea, mentioned 

 ante, p. 22, and referred to the Naugus Head Series. The 

 variety of rocks exposed here — granite, a petrosilicious rock, 

 and a basic rock, probably diabase — is favorable to the view 

 that they form part of a considerable area of crystallines, — 

 perhaps an extension of the rocks of Nahant or Saugus, — 

 rather than a limited mass intersecting slate or conglomerate. 

 In this hypothetical crystalline area, if admitted, we can 

 find an adequate explanation of the south-easterly strike of the 

 Somerville slates, which, it will be observed, show a marked 

 divergence from the slates in Maiden, as if this rock in its east- 

 erly extension divided to pass on opposite sides of a wedge- 

 shaped crystalline area tapering toward the west ; the Medford 

 and Maiden slate bounding it on the north and north-west and 

 the Somerville slate on the south-west. It is less easy to ques- 

 tion the validity of this explanation when we reflect that gener- 

 ally throughout the Boston basin the strike of both the con- 

 glomerate and slate is parallel with the adjacent crystalline 

 border, and that the high and irregular S.S.W. dips of the 

 slate along the north-east side of Somerville indicates their prox- 

 imity to such a border. 



JVahant. — The bold cliffs in the vicinity of East Point, 

 Nahant, are composed of a very dark gray, almost black, 

 compact slate, with a strike N. 60° E., and a nearly uniform 

 northerly dip of about 40°. The general characters of this 

 slate have been noticed on p. 195. The intersecting dykes are 

 large and numerous, cutting the slate both with and across the 

 strike. They are undoubtedly derived from, i.e., are a part of, 

 the extensive formation of coarse, exotic rock of the Naugus 

 Head Series bounding the slate on the north-west. As ex- 

 plained on the page cited above, the slate, especially in the 

 vicinity of the dykes, has suffered considerable change; the 



