205 



six hundred feet (traverse measure) of slate, but no conglom- 

 erate ; the part of the section where this rock might be expected 

 to appear affording no outcrops. A generalized section along 

 ^the north side of the cove is shown in PL 4, fig. 2. 



The evidence is plain : ( 1 ) That the bulk of the conglom- 

 erate in these sections belongs below the slate, thin beds some- 

 times alternating with the slate, near the contact. (2) That 

 the rocks are cut by transverse and strike faults, one or two 

 of the former being particularly evident, producing a lateral dis- 

 placement of several hundred feet ; while the downthrow of the 

 latter has been mainly on the west, concealing the conglom- 

 erate and diminishing the apparent thickness of the section. 

 (3) That the amygdaloid is in part exotic, though elevated chiefly 

 through the agency of faults. Portions of the slate on this 

 shore have a pretty fair cleavage, which, according to the rule 

 in the Boston basin, trends east-west, with a steep dip to 

 the north. Following the slates to the north side of the amyg- 

 daloid area, the strike is observed to change abruptly to nearly 

 east-west, conforming with the border of the amygdaloid. A 

 traverse from the amygdaloid northward, or toward Slate Island, 

 shows only slates, dipping steeply in that direction and considera- 

 bly contorted, all the way to the shore. The conglomerate is here 

 concealed by a fault. The conglomei^ate and slate on the dif- 

 ferent sides of this mass of amygdaloid aiford a good example 

 of quaquaversal dips. 



Recent observations have shown that the narrow south-western 

 angle of the amygdaloid, and the anticlinal which it represents, 

 are continued nearly, if not quite, to Weymouth Back River. 

 The trend of the anticlinal gradually shifts toward east-west, 

 80 that where it crosses Beal Street the strike is N. 60'^ E. 

 The anticlinal appears to be much faulted, for the conglomerate 

 and amygdaloid are greatly mixed up, but this might result in 

 part from the extravasation of the latter. South of Hewitt's 

 Cove the slate is not exposed on the north-west side of this anti- 

 clinal, but on the south-east there are numerous outcrops, and 

 its position is at all points very clearly synclinal. On the 



