]N"o. 7. — The Great Dike at Hough's JVecIc, Quincy, Mass. By John 



Eliot Wolkk 



Tins dike is situated some two and a lialf miles northeast from 

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

 tinuing, with interruptions, for ahout 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 ifmygdaloidal, 

 and sometimes porphyritic, and includes masses whicli resemble fclsitc. 

 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. 17G.) 

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

 Island Cove, there are prominent ledges of conglomerate flanl^ing a large 

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



^ 



latcd bands, due to extravasation or faulting. The conglomerate strikes 

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

 dipping away from the amygdaloid. It holds unmistakable pebbles of 

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

 the north, over the area marked as slate, the rocks arc all concealed by 

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

 of a passage to slate." (p. 200.) The amygdaloid, constituting a mem- 

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

 overlying Primordial conglomerate, and as a sedimentary I'ock 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 cast, among the 

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

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

 a small creeli: 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. Tapers, Bost. Soc. ]N[at.IIist., III., 1880. 



■ 





VOL. VJl. 



xo. 7. 



