175 



Other islands in the breccia are formed of conglomerate, 

 small remnants of a once continuous sheet which erosion has 

 failed to entirely remove. Mr. F. W. Very and I have ob- 

 served exposures here in which the three formations — petro- 

 silex, breccia, and conglomerate — can be clearly seen resting 

 in uncomformable position one upon another in the order 

 named. Although the chronological distinctness of the breccia 

 and the most of the conglomerate does not admit of question, 

 yet it is not always easy to separate them in the field.' For 

 example, in Mount Hope Cemetery, near the gate on the Back 

 Street side there are ledges of a recomposed felsitic rock 

 which belongs stratigraphically with the conglomerate, while 

 lithologically it is a good breccia. 



On the New York and New England R.R., in the vicinity 

 of Bird's Lane, there is a rock transitional in character be- 

 tween the felsitic breccia and the amygdaloid. On the south 

 it is in contact with slate and conglomerate ; and the latter is 

 largely composed of pebbles of this rock and the breccia. ' The 

 breccia north of the amygdaloid in Mattapanis mostly compact, 

 and light-colored, weathering white. Farther north-east, on 

 Washington Street, the rock has a more normal texture, but 

 the small area marked here is based on a single ledge protrud- 

 ing through the conglomerate. South of the Neponset the 

 general character of the breccia, which is small in amount, is 

 the same as in Hyde Park; sometimes, however, the bedding 

 is unusually distinct and the rock takes on a very slaty aspect. 

 The amygdaloid agrees very well with that of Needham, being 

 usually epidotic or chloritic, rarely amygdaloidal, and showing 

 decided approximations toward the more compact of the breccias. 

 At several points where it meets the conglomerate the evidence 

 of a slight degree of fluency and extravasation is very plain. 



Shawmut Group in the South Shore District? — Distinct 

 breccia is not certainly known to exist in this sixth and last 



* For a description of the massive, flinty slate, and allied rocks, occurring as an island 

 in the Blue Hill granite and cropping at several points along the northern border 

 of this range, see ante, p. 37. 



