71 



Traces of the schistose or imperfectly banded petrosilex have 

 been observed on Marblehead Rock, Half-way Rock, and North 

 Gooseberry Island. On Marblehead Rock, especially, it passes 

 into the compact porphyritic varieties next to be described. On 

 Half-way Rock it is of a brownish-red color and the structure is 

 very indistinct ; and, since it here contains a few small pebbles 

 and irregular amygdaloidal cavities, it must be regarded as 

 possibly belonging to the Shawmut group. On Marblehead 

 Neck, at the large outcrop west of Castle Rock, and also at the 

 northern end of the Neck, is a grayish rock, very porphyritic 

 and weathering white, which contains a few small, black and 

 ash-colored petrosilex pebbles, numerous grains of vitreous 

 quartz, and shows, occasionally, distinct traces of the schistose 

 structure ; the layers having precisely the same texture and 

 general appearance as in the typical schistose petrosilex. It is 

 with hesitation that I put this rock down as petrosilex ; and 

 yet I am unable to regard it as anything different. A few small 

 patches have the aspect of coarse granite, though appearing 

 too ill-defined for pebbles. 



Among the more normal types of petrosilex in this Marble- 

 head region, perhaps the least important is the compact, homo- 

 geneous, little porphyritic variety. It is devoid of visible 

 structure, and is not an abundant rock, although occurring in 

 small patches at many points. The principal colors are black, 

 grayish, red, and brown. The schistose petrosilex passes into 

 this variety when extravasated ; in fact when dark-colored 

 this rock usually has the aspect of an exotic, at least as regards 

 its petrologic relations. It occurs on Half-way Rock, where 

 the colors are red and brown, and the Breakers south of Baker's 

 Island are entirely composed of it. The black and apparently 

 exotic portions of this rock on the Breakers, South Gooseberry 

 Island, and Marblehead Rock and Neck, sometimes hold scat- 

 tering pebbles, as already explained in connection with the 

 schistose variety. The percentage of silica in the black rock 

 on the Breakers, as determined by one analysis of an average 

 specimen, is rather low for petrosilex, 70.3. 



