95 



petrosilex in Medford. Here the discontinuity of the granitic 

 belt permits the expansion of the petrosilex northerly through 

 Melrose and Saugus into Wakefield. The petrosilex in Lynn 

 is entirely disconnected from that in the Marblehead region by 

 extensive masses of eruptive granite ; but that they were 

 formerly continuous is in the highest degree probable. It is 

 very evident that the petrosilex of Marblehead Neck and the 

 adjacent islands forms but one area. These are geographic, 

 but not geologic islands. The length of this petrosilex belt, 

 from South Natick to Half Way Rock, is thirty-three miles. 

 An outlying patch of petrosilex is said to occur on the Man- 

 chester shore, on the line of this belt. The north-east and 

 south-west trend is conspicuously marked in the petrosilex 

 areas of Needham, Hyde Park, and Milton. In Dedham and 

 Dover the petrosilex partakes of the complexity which appears 

 to reign in that region. If the Shawmut and Primordial rocks 

 were removed, we should probably find that these isolated 

 masses of petrosilex to the south-west of Boston are really the 

 beginnings of ranges similar to and parallel with that above 

 described. In fact, it is likely that the petrosilex in central 

 and southern Needham is a detached portion of the Natick and 

 Marblehead belt. The remarkable chain of islands stretching 

 from S quantum to the Outer Brewster is directly in line with 

 the petrosilex areas in Hyde Park and northern Milton, and 

 certainly suggests their extension in that direction. 



Relations of the petrosilex and granite. — After many 

 years of field work, Prof. Edward Hitchcock summed up his 

 observations on the relations of the granite and petrosilex in 

 the following language : * " The porphyry (petrosilex) is 

 associated, both on the north and south of Boston, with syenite 

 (granite) ; and in all cases, so far as I have observed, the 

 porphyry lies above the syenite, and there is a gradual tran- 

 sition between the two rocks. This fact is most obvious in 

 the Blue Hill Range, where one is often much perplexed to 



1 Final Report on the Geol. of Mass., p. 667. 



