171 



enveloping the Huronian petrosilex in Needham, and send- 

 ing tapering, finger-like extensions north-easterly through the 

 conglomerates of Newton, Brookline, and Brighton, and south- 

 westerly into Natick, affords breccia very locally at two or 

 three points only. These may be conveniently described 

 first. In Newton, south of Newton Centre and east of Upper 

 Falls, in the angle between Dedham Street and Boylston 

 Street, and along a new street connecting these two, I ob- 

 served the foUoAving section from south to north : (1) The 

 usual slaty amygdaloid of that region; (2) A very firm, in- 

 distinct, and slaty-looking conglomerate or breccia, holding 

 pebbles of petrosilex (several varieties), granite, diorite, and 

 quartzite, and with a granitic, dioritic, felsitic, or slaty paste ; 

 the pebbles are sometimes wanting, when the rock becomes a 

 slaty felsite. In my mind, some doubt attaches to the Shawmut 

 age of this conglomerate. (3) Primordial conglomerate of the 

 usual type, holding pebbles of amygdaloid, and dipping to the 

 north. (4) A true breccia containing fragments of dark- 

 colored petrosilex and eurite, which are mostly small, and 

 rather rare, with a greenish-white felsitic paste. This is suc- 

 ceeded without any marked line of division by (5) a dark 

 purplish rock, which, though in the main of quite compact 

 appearance, is in part a distinct and very irregular breccia, 

 holding angular masses of quartzite and petrosilex, while in 

 other portions there is a manifest tendency to develop an 

 amygdaloid structure, and still other parts are decidedly 

 porphyritic. It is apparently a transition type between the 

 compact rocks of the breccia division and the amygdaloid. 

 Overlying this is (6) Primordial conglomerate, most of which 

 is exceedingly coarse. The conglomerate (3) appears to form 

 an isolated superficial patch between the two masses of Shaw- 

 mut rock. 



In South Natick a band of stratified rocks follows the course 

 of the Charles River and Elm Street from the Needham boun- 

 dary nearly, if not quite, into Sherborn. The strike is N.E.— 

 S.W. ; and the whole series has steep north-westerly dips. Two 



