237 



from modern sea-water, agrees closely with the water obtained 

 by similar wells from the Cambrian strata of Canada, — a fact 

 which points to the Cambrian age of our Boston slate. Slate 

 has also been encountered in the bottom of a well eighty or 

 ninety feet deep at the corner of Brookline Street and Harrison 

 Avenue, and in a well one hundred and seventy feet deep at 

 No. 791 Tremont Street. ■ 



Assuming, as I think we may, that a broad band of slate 

 underlies the districts mentioned in the preceding paragraph, 

 including Governor's Island, and tracing it toward the west, we 

 find that its breadth is diminished rapidly where it passes like a 

 wedge between the immense masses of conglomerate on the 

 north and south. There is conglomerate in Brookline, on Cy- 

 press Street, north of the New York and New England Rail- 

 road, and on the south side of this railroad nearly one-half 

 mile east of Reservoir Station. Conglomerate and amygaloid 

 form extensive ledges south-west of the crossing of Brighton 

 and Harvard Streets in Brighton ; the former rock outcrops on 

 the north-west side of Corey's Hill, and almost continuously 

 from this point to where Beacon Street meets the Chestnut 

 Hill Reservoir. In the vicinity of the reservoir the breadth of 

 the slate band cannot exceed one-fourth of a mile. On Beacon 

 Street, where it crosses the Brighton and Brookline boundary, 

 the thin-bedded, gray and brown slate has a variable, mostly 

 gentle, dip to the north. At the crossing of Brighton Street 

 and the New York and New England Railroad, Reservoir Sta- 

 tion, the same slate, mostly grayish, inclining to black, and 

 somewhat contorted, has a northerly dip of 25°-30°. The 

 magnificent section transiently exposed during the construction 

 of the Chestnut Hill Reservoir has been described by Prof. N. 

 S. Shaler ^ as follows : " On the south border of the lower res- 

 ervoir at Chestnut Hill there is an outcrop of rocks in all 

 important respects closely resembling the Cambridge slates. 

 These are traceable for a distance of about seven hundred feet 

 in a northerly direction across the floor of the reservoir, having 



^Proo. Boat. Soc. Nat. Hist., xiii., 176. 



