200 



dip, but is also underlaid by the same rock ; which penetrates the 

 sandstone in a manner possible only with an eruptive. The 

 great uniformity in the dip and strike of the conglomerate 

 over this entire area of a square mile or more, however, appears 

 inconsistent with the extravasation of much of the amygdaloid. 



On the map I have attempted to trace the distribution of the 

 different rocks in this Nantasket district only in the most gen- 

 eral way. The exposures are uncommonly good, but the areas 

 are too small and complicated for accurate representation on 

 this scale. There is more amygdaloid on the west side of the 

 road running south from the beach than I have indicated. This 

 rock here includes nearly every variety known to occur in the 

 Boston basin, but there is a larger proportion of the typical 

 amygdaloid than I have observed elsewhere. The conglomerate 

 farthest to the east, near the island of Black Rock, is coarse and 

 reddish ; consisting chiefly of the large grained, pinkish granite 

 of the adjacent mainland, in well-rounded masses of all sizes up 

 to a foot or more in diameter ; but it also includes pebbles and 

 boulders of amygdaloid. The whole rock is thoroughly indu- 

 rated, though this is most noticeable in the finer layers. West 

 of the road mentioned, the conglomerate is also sometimes 

 quite coarse ; and the colors are reddish and gray, rarely green- 

 ish. The principal constituents are granite, petrosilex, and 

 amygdaloid. Intercalated layers of red sandstone show that 

 the dip already stated characterizes the entire area. Igneous 

 agency has given portions of the sandstone the hardness of 

 jasper. In the small bay immediately south of the steamboat 

 wharf, the conglomerate can be seen reposing upon the amyg- 

 daloid ; and it is here a very coarse rock, holding masses, often 

 angular, one to two feet in diameter, of granite and petrosilex 

 and of the very same amygdaloid which it overlies. The rocks 

 on the north-east side of Planter's Hill, in Hingham, are a 

 continuation along the strike of those in the Nantasket area, 

 and the relations are the same. 



It is impossible to determine the thickness of the conglome- 

 rate in this district ; but it is probably not less than three or 



