77 



purple. It rarely holds pebbles, and might be described as de- 

 void of all traces of sedimentary structure but for the limited 

 patches of banded petrosilex above described, which occur in it, 

 and differ from it only in being banded. We have no reason 

 to doubt that this rock is mainly petrosilex. A characteristic 

 brownish-red specimen from Lynn contains 73.9 per cent, of 

 silica. Towards the north, in Lynn, the rock varies much, is 

 frequently of a gray or drab color ; and near the reservoir, on 

 the road to Dungeon Rock, it holds grains of quartz as well as 

 feldspar. Where the tongue of granite penetrates the petro- 

 silex on the west side of Wenuchus Lake, these two rocks 

 appear to have reacted upon each other, producing mutual 

 modifications, so that the contact is now marked by a zone of 

 debatable ground, in which one seems to find all sorts of transi- 

 tions between petrosilex and granite ; and it is clear that one, 

 perhaps both, of these rocks must have been fluent. 



Of the same general character — frequently quite compact, 

 and never very porphyritic — is the most of the petrosilex in 

 the north-west corner of Saugus and the adjacent portions of 

 Wakefield and Melrose ; also in Medford, north of the Naugus 

 Head diorite, where it shows frequent local approximations to 

 granite ; in short, this is the prevailing, the characteristic va- 

 riety of petrosilex for the entire Lynn and Medford area. A 

 grayish, non-porphyritic specimen from Maple wood gave 72.6 

 per cent, of silica. At many points in North Saugus, between 

 Main Street and Saugus River, the rock is more than usually 

 porphyritic, yet it holds numerous pebbles, and may possibly 

 belong with the breccia ; this, however, is improbable, since 

 the undoubted breccia contains pebbles of this pebble-bearing 

 petrosilex. Are there two breccias here? I think not, although 

 the appearances are not wholly inconsistent with that view. 



Along Fulton Street in Medford, between Salem Street and 

 the granite on the north, there is a large area of a dioritic rock 

 which appears to alternate between granite and felsite. It 

 is composed mainly of pinkish and greenish feldspar, with some 

 hornblendic material which, inconspicuous and slaty-looking, as 



