236 



are prominently placed on the east side of the railroad immedi- 

 ately north of Roxbury Street ; but north of this, and of a line 

 from here to Mt. Vernon Street in Dorchester, everything is 

 concealed. The fact that at Mt. Vernon Street the con- 

 glomerate has scarcely begun to show a northerly dip, and the 

 abundance of the loose masses of conglomerate scattered over 

 the extreme northern end of Dorchester, convince me that here 

 at least the border of this rock runs farther north than indicated 

 on the map. 



According to Mr. W. W. Dodge, ^ there was formerly a 

 quarry of slate in the north-west part of South Boston, near 

 the corner of Fourth and E Streets. A dark-colored, homo- 

 geneous slate is now exposed on F Street, north of Broad- 

 way, dipping north in. the neighborhood of sixty degrees. 

 I was directed to this exposure, which appears to be the only 

 one now existing in South Boston, by Mr. Henry Richards. 

 A "slate ledge," it is well known, exists in the shallow 

 water off the north side of South Boston. One and a fourth 

 miles east of this sunken ledge there is compact gray and 

 grayish-black slate on Governor's Island. I have not visited 

 this island, but, according to the labels on specimens collected, 

 and given me, by Prof. W. B. Rogers, the rock is much dis- 

 turbed, the strike varying from N. 80^ W. to N. 10° E., a 

 range of ninety degrees. There are no natural exposures of 

 the rocks in East Boston or Boston proper ; but the artesian 

 well of the Boston Gas Company, on Causeway Street, 1,750 

 feet deep, was bored almost wholly in slate, the last fifty feet 

 only being in a harder rock, apparently crystalline, though this 

 may be conglomerate. Of course the nearly seventeen hundred 

 feet through slate affords no clue to the thickness of that rock at 

 this point, farther than to determine a maximum limit, for the 

 beds are probably highly inclined, the well cutting them very 

 obliquely. Dr. T. Sterry Hunt has shown, ^ that the water 

 from this well, while ditferin_g widely in chemical composition 



iProo. Bost. Soc. Nat. Hist., XTii., 411. 

 ^'Proo. Bost. Sue. Nat. Hist., xvn., 486. 



