74 



ite cutting through the Huronian diorites. Here the base is of 

 different shades of drab, red, and brown ; and here, also, the 

 quartz is crystalline, and the crystals of both it and the feldspar, 

 which is pink and white, varying with the base, are quite large 

 and very distinct. A rock very similar to this occurs on the 

 Beverly shore, in the bay between Woodberry's Point and 

 Hospital Point. It forms several large masses, and is cut by 

 the coarse granite found here, and also by the feldspathic dio- 

 rite of the Naugus Head series. This elvanite is associated with 

 the granulite noticed ante, p. 35. The structure of the latter 

 rock is very similar to that of the schistose petrosilex on Marble- 

 head neck, the principal difference being that the feldspathic 

 matrix is more crystalline and the schistose layers more dis- 

 tinctly quartzose ; the granulite, also, contains traces of mica- 

 ceous material. 



Petrosilex of the Lynn and Medford Area. — The 

 petrosilex in Lynn, Saugus, Wakefield, Melrose, Maiden, 

 and Medford, would probably be found continuous, or nearly 

 so, but for the patches of Shawmut breccia reposing upon it, and 

 hence it is proper to regard this as one large irregular area. 

 Many of the varieties occurring in the Marblehead region are 

 found here ; but they have not in every case the same relative 

 importance, so that the general aspect of the formation is 

 different in the two areas. The normally banded petrosilex is 

 rare in the Lynn and Medford region. Prof. Hyatt's collection, 

 however, contains specimens of it from several points in Lynn, 

 and I have observed a rock showing traces of this structure be- 

 tween North Saugus and Saugus Centre, forming a high ridge 

 along the east side of the road from the former place to Lynn. 

 A specimen from this Saugus locality yielded 76 per cent, of 

 silica, from which we may safely infer that this rock is a true 

 petrosilex. The schistose variety is more abundant. It is 

 usually of different shades of red, brown and purple ; and the 

 structure is, for the most part, very fine, with now and then a 

 coarse layer. It is even more evident here than on Marblehead 

 Neck that this structure cannot have resulted from the altera- 



