168 



represented by the breccia division alone. This division, how- 

 ever, is here exceedingly heterogeneous, comprising rocks of 

 all textures, from an impalpable felsite to a coarse breccia 

 holding fragments several inches in diameter. The paste is 

 always felsitic, and is usually quite compact, though sometimes 

 developing feldspar crystals ; these appear to be mainly, if not 

 wholly, triclinic, and are sometimes so numerous as, when the 

 pebbles are small or the paste relatively abundant, to give the 

 rock a decidedly porphyritic aspect. With comparatively rare 

 exceptions the fragments are some variety of petrosilex, such as 

 is indigenous in this region ; in a few cases they are granitic, 

 while others appear to be derived from the feldspathie rocks of 

 the Naugus Head series. Magnetite is rarely, if ever, wanting 

 in the Marblehead breccia. It is found chiefly disseminated in 

 minute grains in the paste, seldom occurring in the pebbles ; 

 and it is most abundant, or at least most noticeable, in the rocks 

 of medium fineness, — the grits and sandstones. Since this 

 mineral is of rare occurrence in the petrosilex and felsite I con- 

 clude that it may have come in part from the Naugus Head series. 

 Pyrite is also a common mineral in certain of the breccias ; it 

 occurs in much the same way as the magnetite, but less abun- 

 dantly, and it has more the appearance of being indigenous. 



On Marblehead Rock there is a distinctly stratified, non- 

 porphyritic felsite, or felsitic slate, alternating with sandstones 

 and grits. These beds are much broken and twisted by the 

 petrosilex in which they are enveloped, and which is eruptive 

 through them. Approximate strike, east-west; dip, southerly. 

 Lowell's Island is largely composed of a firm feldspatliic sand- 

 stone of a normal and very uniform texture. Strike, N.E.— 

 •S.W. ; dip, south-east. It is on this island that the Shawmut 

 rocks approach nearest, geographically, to the Naugus Head 

 series ; and it is interesting to observe that this feldspatliic sand- 

 stone is especially rich in magnetite. Small-pebbled breccia, in 

 which the paste largely predominates, and which is often dis- 

 tinguishable with difficulty from petrosilex, occurs on the Goose- 

 berry Islands. On Marblehead Neck a true breccia occurs at 



