176 



of the districts characterized by the Shawmut group ; but the 

 amygdaloid is found in abundance at various points, and presents 

 many varieties. On Hough's Neck, in Quincy, the amygdaloid 

 is a green, slaty rock ; it is sometimes amygdaloidal, and some- 

 times 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 erup- 

 tive. In Hingham, along the railroad, the amygdaloid appears 

 as a narrow band separating the granite and conglomerate. Im- 

 mediately north and west of the petrosilex in this town it is, in 

 part, typically amygdaloidal. Farther north, in the vicinity of 

 Hewitt's Cove, there is a considerable area of this rock, which 

 here embraces nearly every variety that has been noticed in the 

 preceding descriptions. The relations of the amygdaloid to the 

 conglomerate and slate are well displayed along the shore of 

 the cove. 



To the Shawmut amygdaloid must be referred that great 

 mass of amygdaloidal and nondescript rocks so intimately as- 

 sociated with the conglomerate in the southern part of Hull, 

 south of Nantasket Beach. , The relation to the conglomerate 



« 



is similar to what obtains in Brighton, though, perhaps, on the 

 whole, more complicated* The amygdaloidal character is very 

 frequently and perfectly developed, especially in the southern part 

 of the area. Toward the north the rock is chiefly compact and 

 epidotic or chloritic ; large masses, however, hold much quartz 

 — crystalline and chalcedonic — in irregular veins, and have a 

 brecciated aspect. In this direction, too, traces of bedding and 

 an arenaceous texture are visible. On the island of Black 

 Rock, off the Cohasset shore, the prevailing rock shows a dark, 

 apparently compact, base, holding crystals of triclinic feldspar. 

 This passes into a less porphyritic, stratified rock of the same 

 dark color, but holding rounded grains of epidote, and evidently 

 inseparable from the amygdaloid. Irregular masses are green- 

 ish-white, compact and epidotic. 



