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quent to the development of the schistosity ; and, since it is in the 

 highest degree probable that all the fragments are of the same 

 age, we find it necessary to conclude that, while those composed 

 of petrosilex are most likely the result of internal fracture and 

 crushing, the granitic and dioritic fragments must have been 

 introduced from without, apparently while the rock was in a plas- 

 tic state, and before the segregation of the quartz had ceased. 

 The numerous dislocations and the local crushing in situ of 

 portions of this rock are illustrated, though on a small scale 

 and rather inadequately, in fig. 1. In this specimen the schis- 

 tosity is unusually fine. The fact that fragments of the red 

 petrosilex are never found in the gray, and vice versa, greatly 

 strengthens the view that the petrosilex fragments have origi- 

 nated in or near their present positions ; for, if they were of foreign 

 birth, this relation to the adjoining rock would be very difficult 

 to explain. It is natural, at this juncture, to compare the con- 

 centric lenticular layers of this petrosilex with the layer-like 

 concretions in the banded petrosilex west of Back Street. 

 But there are conspicuous differences, the true concretions 

 being broadly and smoothly rounded on the margins, and 

 presenting everywhere rounded botryoidal surfaces, very unlike 

 the ragged outlines of the schistose layers. I am still obliged 

 to regard the latter as probably resulting primarily from some 

 peculiarity in the mode of original deposition, modified subse- 

 quently, in most cases, by a process of segregation which shar- 

 pened the outlines of the layers and developed the quartzose axes . 

 This schistose petrosilex, both red and gray, occurs very 

 locally at several points in Hyde Park Village, and along the 

 New York and New England R.R., between Mattapan Station 

 and Oakland Street ; also near the end of Bird's Lane west of 

 the railroad. In some places it either holds so many pebbles 

 as to be nearly indistinguishable from breccia, or else the brec- 

 cia contains an occasional flattened layer, derived, perhaps, 

 with the rest of the material, from the petrosilex. Along the 

 west side of the railroad, near Mattapan Station, there are 

 several very fine exposures of the contact of the schistose petro- 



