80 



from which, and its great uniformity, I infer that this rock is 

 all petrosilex. The vertical joint planes in this rock frequently 

 intersect in such a manner as to develop a beautiful and j^erfect 

 columnar structure. The columns are not uncommonly hex- 

 agonal, four to eight inches in diameter, eight to twelve feet or 

 more in length, and as true and regular as any in basalt. Sev- 

 eral fine examples of this structure are exposed in the cuts on 

 the New York and New England Railroad, north-east of Charles- 

 River Village. 



Petrosilex in Dover, Medfield, and DedhaTU. — The two 

 small patches of petrosilex along the railroad in Dover and 

 Medfield exhibit local transitions toward granite and diorite. 

 The rock is of greenish and grayish hues. My data from this 

 region are meagre, and further exploration would probably dis- 

 cover more of these small islands of petrosilex among the 

 granite and diorite. The petrosilex of the large area in Dover 

 and West Dedham presents many varieties. As a rule it 

 becomes more crystalline toward the west, appearing to pass 

 insensibly into the granites which border it in that direction. 

 On the other hand, we are as yet unable, as already stated, to 

 separate the petrosilex from the breccia on the south. The 

 difficulty here arises from the enclosure of pebbles in portions 

 of the petrosilex adjacent to the breccia ; and the question is un- 

 settled as to whether the explanation found adequate on Marble- 

 head Neck will fit the similar phenomena in West Dedham. 

 The magnificent exposures in the former locality greatly facili- 

 tated the determination of the relations of the rocks. The 

 main body of the petrosilex in the vicinity of the breccia is of a 

 greenish hue, waxy lustre, usually not very hard, and, save when 

 holding pebbles or an occasional grain of quartz, quite compact. 

 It seems to have the composition of petrosilex, an average speci- 

 men from the hill west of the " Oven Mouth," on Main Street, 

 giving 72.35 per cent, of silica. Yet its inferior hardness, 

 usually yielding to the knife, suggests recomposition, though it 

 may mean simply decomposition. It does not differ in this 

 respect, however, from much of the elvanite in Needham. 



