52 



poses directly upon the latter ; and both series have evidently 

 suffered great disturbance. The light-colored Shawmut breccia 

 is cut and torn in every direction by tortuous dykes of the 

 black petrosilex, which itself very clearly holds angular pebbles 

 of different varieties of petrosilex. Many of these pebbles are 

 of the same general character as the enclosing rock, often ap- 

 pearing to be identical with it, and as a consequence very 

 inconspicuous ; but those which first arrest the observer's eye, 

 including a large proportion of the whole, are a highly crys- 

 talline, whitish variety of petrosilex which is precisely similar 

 to that forming large ledges a few rods to the northward. The 

 main point to be proved in this connection is brought out at 

 this locality with especial clearness; viz., that the very same 

 petrosilex which holds pebbles underlies, and is in part eruptive 

 through, the breccia, forcing the conclusion that, in spite of a cer- 

 tain superficial resemblance, these are distinct formations. This 

 interesting locality is more fully described on a subsequent page. 

 There are other localities where the evidence is nearly as 

 plain, and points to the same conclusion. Among these are 

 the vicinity of Cliftondale Station on the Saugus Branch of the 

 Eastern Railroad, West Dedham, and Hyde Park. At Clifton- 

 dale and in Hyde Park, the majority of the included fragments 

 are of precisely the same character as the enclosing petrosilex, 

 and are sufficiently accounted for by the internal crushing and 

 regelation theory; but other blocks — of quartzite, diorite, and 

 granite — must have been introduced from without, and imply 

 some plasticity in the petrosilex at the time of their enclosure. 

 To summarize, the facts observable at the places named, and 

 elsewhere, compel us to suppose that the petrosilex, while suf- 

 fering great disturbance, has been locally crushed and brecciated, 

 and that certain portions of this rock, perhaps as a consequence 

 of enormous friction, have been softened to an extent that 

 would permit the envelopment of extraneous masses. In say- 

 ing this I am conscious that such inferences must be drawn 

 with the utmost caution, if we would avoid mistaking petrosi- 

 liceous breccia for brecciated petrosilex. 



