210 



fig. 4) : first, at least three hundred feet of the grayish-black 

 and frequently indurated slates, already mentioned, with a 

 nearly vertical southerly dip. On the south the slate appears 

 quite near the granite, while the most northerly outcrop comes 

 within, perhaps, three hundred feet of Adams Street. Beyond 

 this the section is concealed almost to the brook ; and then 

 coarse-pebbled conglomerate appears for about two hundred and 

 fifty feet, passing into sandstone on the north. The sandstone 

 continues for sixty or seventy feet, gradually changing to slates, 

 of which about one hundred and fifty feet are exposed. The 

 normal strike for the entire section is east-west, or N. 80° E., 

 and the slate on the north has a high dip in that direction. But 

 the bedding is much disturbed locally by masses of eruptive 

 diorite ; so that, at some points, the dip is reversed, and the 

 strike shifted to N. 65°-70° E. The slates at the north end 

 of the section are apparently the same as those south of the 

 brook, but, being farther from the granite, are less indurated. 

 This sandstone and slate are in the same part of the section, 

 i. e., on the same line of strike, as the similar beds along the 

 railroad, which are also north of the brook. The conglomerate 

 is largely composed of granite, but holds pebbles of petrosilex, 

 amygdaloid, and, occasionally, of breccia. It can be traced to 

 the west, by almost continuous outcrops, for nearly one-half 

 mile, occurring, in this direction, wholly on the south side of 

 the brook. 



About one hundred and twenty-five rods west of the section just 

 described, and nearly one-fourth mile east of where Adams Street 

 crosses the brook, I have measured another section of this 

 Black's Creek and Rock Island Cove anticlinal (PI. 4, fig. 5). 

 From the main range of granite northwards, on this line, we have 

 the following : — 1 , granite ; 2, the narrow belt of greenish, semi- 

 crystalline rock referred to the Shawmut group, and apparently 

 cut by granite ; 3, granite, not indicated on the map, extending 

 to the north side of Adams Street ; 4, conglomerate, at least 

 six hundred feet, traverse measure, reaching nearly to the 

 brook. The junction of the granite and conglomerate is about 



