56 



general trend of the belt is E.N.E. Its width is very vari- 

 able, a few rods to one and a half miles ; this results mainly 

 from the irregular manner in which the newer rocks — Shaw- 

 mut and Primordial — are superimposed upon the petrosilex. 

 Leaving these more recent rocks out of view, the petrosilex 

 appears to be bounded on all sides by granite ; lying between 

 the Rowley granitic belt on the south and the Newbury belt on 

 the north. This petrosilex is, on the whole, remarkable for 

 its uniformity. The prevailing color is a deep red, or brownish- 

 red ; but it is sometimes purplish, pinkish, or grayish, and 

 occasionally nearly white. It is never distinctly porphyritic, 

 but nearly always has a well-marked banded structure. To the 

 north of Old Town Hill and between that and Kent's Island, 

 however, much of the petrosilex is not banded. Its colors here 

 are different shades of red and gray, varying from a dirty white 

 to nearly black, but all weathering reddish. Some of it is dis- 

 tinctly quartzose and has a granular or euritic aspect. The 

 banded structure so characteristic of most of the Newbury 

 petrosilex is produced by the interlamination of layers of 

 quartzose and feldspathic materials. The quartzose or jas- 

 pery layers are darker colored than the feldspathic, and the 

 latter are prone to weather white ; so that the banding is most 

 conspicuous on weathered surfaces. The thickness of the lami- 

 nae usually varies from a fine line to one-sixteenth of an inch, 

 and seldom exceeds one-eighth of an inch. The individual lami- 

 nae are generally continuous for several feet or yards, except 

 where broken by faults or other disturbance ; and adjoining 

 laminae, at least the larger ones, rarely coalesce or divide. 

 As a rule, each layer maintains the same thickness through- 

 out its length ; the shorter ones, however, are sometimes 

 lenticular, and these occasionally coalesce. The petrosilex 

 has been immensely disturbed, and consequently the laminae 

 strike and dip in every possible direction ; yet there are a 

 certain general strike and dip which predominate and are 

 characteristic of the rock as a whole. This prevailing strike 

 ranges from N.E.-S.W. to E.-W., parallel with the general 



