67 



been softened in situ, and, while portions of the plastic mass 

 were extravasated through the breccia, the remainder continued 

 in a more or less tranquil state. The clearly extravasated por- 

 tions of the petrosilex are charged with a greater number and 

 variety of pebbles than the relatively undisturbed portions 

 already described. Yet these exotic masses never come to 

 resemble the breccia. The latter rock is always light-colored, 

 while the former is black ; hence the contrast is great and the 

 contacts are very distinct, and easily traced. 



This variety of petrosilex, in its origin and relations, has 

 been justly regarded as presenting one of the most difficult 

 problems which this, geologically, very complex locality affords. 

 The explanation proposed above, in which this rock is regarded 

 as essentially stratified, its structure differing in degree only 

 from the more normal stratification of the regularly banded 

 petrosilex, as shown by the transitional types of structure ; and 

 the enclosure of pebbles accounted for by envelopment while 

 in a plastic state, the softened condition being demonstrated by 

 the exotic nature of portions of the rock, differs widely from 

 any that has been advanced heretofore. Yet I am convinced 

 that this is the only hypothesis affording a rational explanation 

 of all the facts. In the view proposed by Prof. Hyatt, ^ and 

 partially adopted by the present writer in the report on the 

 Centennial Geological Map of Massachusetts, this petrosilex 

 was regarded as originally identical with the breccia, represent- 

 ing a portion of the latter rock which had been so metamor- 

 phosed that the pebbles were flattened out into thin lenticular 

 sheets, each pebble in the breccia giving rise to one of the lay- 

 ers mentioned in the foregoing description of the petrosilex ; 

 while the undistorted, angular pebbles in the petrosilex were 

 considered as having successfully resisted the flattening process, 

 and as proving the original pebbly or breccia structure of the 

 entire rock. This point reached, and the evidence apj)earing 

 irrefragable, it was natural and easy to conceive the pebbles, 

 under continued compression, as becoming still thinner, and 



^ Proo. Bost. Soc. Nat. Hist., xvm., 223. 



