268 



exactly parallel with the slate ; and the remainder, whieh are 

 extremely irregular in Form, hreak aeross the slate approxi- 

 mately at light angles. Although two systems of dikes 

 utay thus be reeognized, as regards mere direetion, they are 

 undoubtedly all of" the same age ; for while they are somewhat 

 peculiar in lithological aspect, they ai'C all extremely alike in 

 this respect. The trap is, without an important exception, a 

 finely and indistinctly crystalline, almost compact, light 

 brownish gray variety, of a dull, ashen luster. Trap of this 

 general character and evidently highly altered, has been 

 observed repeatedly in the thin-bedded slates of this district, 

 and is, perhaps, somewhat peculiar to them. On the north 

 shore some very small dikes (from 1 to 6 inches) not included in 

 the previous enumeration have been altered to impure epidote. 

 The small gray dikes in the slate on the south side of Grape 

 Island are precisely similar to those on Slate Island. 



Dikes ill the Granitic Rocks. 



Mr. Bouve has carefully traced out the dikes of the granitic 

 area, so far as they are exposed on or near the streets, and the 

 following notes are condensed from his paper. 



On Meeting House Hill, Main Street, South Hingham, a 

 few steps north from the church, a dike from 5 to 6 feet wide 

 is exposed in the granite for about 70 feet, trending west- 

 northwest. The bold ledge of granite in the angle between 

 Leavitt and Jones Streets is divided by three prominent east- 

 west dikes. They are readily found by proceeding 700 feet 

 along Leavitt Street on the left side from Weir River to 

 the house of Mr. Alanson Crosby, and then passing to the rear 

 of the house about 300 feet from the road. The most northerly 

 of the three dikes is from 2 to 3 feet in width ; the second, 

 18 feet from the first, is 10 feet wide, and exposed for 75 feet ; 

 and the third, 40 feet from the second, is from 3 to 4 feet wide. 

 On Leavitt Street about a mile and a half from Leavitt's 



