No. 7. — Tlie Great Dike at Rough's Ncch, Quincy, Mass. By John 



Eliot Wolff. 



This dike is situated some two and a half miles northeast from 

 Quincy Depot ; rising, when first seen, as an irregular ridge, and con- 

 tinuing, with interruptions, for about a mile in an easterly direction. 



Mr. Crosby has mentioned this locality in his "Contributions to the 

 Geology of Eastern Massachusetts ":* — "On Hough's Neck, in Quincy, 

 the amygdaloid is a green, slaty rock ; it is sometimes amygdaloidal, 

 and sometimes porphyritic, and includes masses which resemble felsite. 

 It occupies the axis of an anticlinal in the conglomerate ; and also cuts 

 the latter rock very freely, after the manner of an eruptive." (p. 176.) 

 Again: — "On Hough's Neck, in Quincy, along the north side of Rock 

 Island Cove, there are prominent ledges of conglomerate flanking a large 

 mass of amygdaloid, and the latter rock crops through the former in iso- 

 lated bands, duo to extravasation or faulting. The conglomerate strikes 

 about east-west, and shows neaj-ly vertical dips to the north and south, 

 dipping away from the amygdaloid. It holds unmistakable pebbles of 

 Shawmut breccia. This is clearly a faulted anticlinal fold. Toward 

 the north, over the area marked as slate, the rocks are all concealed bv 

 drift ; but on the south the conglomerate shows veiy plain indications 

 of a passage to slate." {p. 209.) The amygdaloid, constituting a mem- 

 ber of Crosby's Shawmut gi'oup, is regarded by him as older than the 

 overlying Primordial conglomerate, and as a sedimentary rock in general, 

 though sometimes presenting evidence of intrusion. 



The country rock of the dike is a coarse conglomerate, with occasional 

 interbedded layers of red sandstone and slate. At the eastern end it is 

 bordered on both sides by the conglomerate. After running for a quar- 

 ter of a mile as a ridge, the dike suddenly loses its ridge character, and 

 occasional exposures only are found in the field to the east, among the 

 outcropping conglomerate ledges. It can be traced thus for a quarter 

 of a mile ; then for some hundred feet no outcrop of dike is found until 

 a small creek is reached. Crossing this, however, we again find a 

 dike continuing as a ridge in the same direction for some hundred yards, 

 when it disappears under the drift of a headland. This exposure, how- 



* Occas. Papers, Bost. Soc. Nat. Hist., III., 1880. 



VOL. VII. — NO. 7. 



