81 



What appears to be the same rock, minus the pebbles, and 

 sometimes more crystalline, covers a large area to the north 

 and west, being the prevailing variety in this region. The 

 pebbles are very rare, except in the immediate vicinity of the 

 breccia, and here they are never so numerous as to give the 

 rock a conglomerate aspect; i.e., as regards texture, there is 

 nothing like a transition from the petrosilex to the breccia. 

 Morover, the breccia holds pebbles of a petrosilex indistinguish- 

 able from that in question. The pebbles are usually petrosilex, 

 but many granitic pebbles have been observed ; these are fine- 

 grained. The petrosilex pebbles are commonly pinkish, and 

 this is the color of many of the pebbles in the breccia. 



At the corner of Main and Pond Streets in West Dedham, 

 and quite near the breccia, there is " flattened pebble " or schis- 

 tose petrosilex, identical, save that it has a reddish tinge, with 

 that studied on Marblehead Neck. It bears no resemblance 

 as regards either structure or component materials to the adjoin- 

 ing breccia. More or less distinctly banded petrosilex is known 

 to occur at several points in this Dedham and Dover, area, but 

 always in small patches. On Dover Street, in Dover, it holds 

 rounded grains of quartz. Toward the east, on and near Fox 

 Hill, we have the most typical petrosilex which this area affords. 

 It is black, hard, and compact, except at one point where Mr. 

 Very has observed banding. The bands are distinct on the 

 weathered surface, but entirely wanting on the fresh fracture ; 

 and, since this petrosilex is very homogeneous throughout, we 

 conclude that it was probably all banded originally. This rock 

 has contributed many pebbles to the breccia. A specimen from 

 Fox Hill has been found to contain 72.9 per cent, of silica, and 

 18.7 per cent, of aluminum and iron sesquioxides. A red, 

 banded petrosilex, apparently more quartzose than the last, 

 occurs in the breccia at the " Oven Mouth." It forms a layer 

 or bed about a foot wide, and seems to be contemporaneous 

 with the enclosing breccia, but it more probably owes its posi- 

 tion to some dislocation of the rocks. It also occurs a mile to 

 the north, in Dover, and probably at other points. 



OCCAS. PAPERS B. S. N. H.— III. 6 



