66 



ture and reconsolidation, as explained ante, p. 50 ; but as a 

 rule they consist of a very crystalline, white petrosilex, identical 

 with a large mass which occurs in situ immediately north of the 

 rock in question. The pebbles are not numerous, — perhaps 

 one of the larger pebbles in a square foot or even in a square 

 yard ; and their distribution is far from uniform. They are 

 scattered about here and there without any regard to system or 

 definite arrangement. A characteristic example is shown in PI. 

 1, fig. 7 ; and here it can be plainly seen that the dark, schistose 

 layers are squeezed and crowded together on either side of the 

 pebble as if to make room for it. Yet in some cases, probably in 

 all, if we could see all sides of the pebbles, it is clear that in one 

 direction from each pebble the schistose structure is greatly dis- 

 turbed, or entirely broken up and destroyed. In brief, the rela- 

 tions of these pebbles to the enclosing rock are precisely such as 

 would result if the pebbles were imbedded in the rock while the 

 latter was in a plastic or semi-fluid state ; and this, I have little 

 doubt, is the true explanation. This rock is very intimately re- 

 lated, petrologically, to the Shawmut breccia occurring here, and 

 it is chiefly this variety of petrosilex to which allusion was made, 

 ante, p. 52, as having been extravasated through the breccia. 

 The exposures are so good as to place the relations of these 

 rocks beyond dispute, and there can be no doubt whatever that 

 portions of the schistose petrosilex have been in a plastic and 

 yielding condition. It is unnecessary to suppose that the ex- 

 -travasation has been great ; that would, indeed, be contrary to 

 the plain facts. In the first place, the breccia at this point 

 evidently reposes for the most part directly upon this variety of 

 petrosilex, pebbles of which enter largely into its composition, 

 showing that this is the normal and original relation of the two 

 rocks. Secondly, the structure of the petrosilex evinces, as 

 already explained, that the disturbance which it has experienced 

 has not been excessive. The schistosity is frequently preserved 

 intact even in contact with the breccia, and as a rule it is wholly 

 lost only where the petrosilex has been forced into narrow, 

 tortuous crevices in the breccia. In fine, the petrosilex has 



