Shaler.] 1 76 [December 3, 



After several years of searching I have succeeded in finding a sec- 

 tion which shows pretty clearly the relation of the Cambridge slates 

 and Roxbury conglomerate. The most important part of this exposure 

 has been uncovered by the extensive excavations now being made in 

 the construction of the Chestnut Hill reservoir. In passing from the 

 southern side of this reservoir in a nearly due north direction, we 

 cross what seems to be a complete section of the slates and conglom- 

 erates of these two series of beds. On the south border of the lower 

 reservoir at Chestnut Hill there is an outcrop of rocks in all impor- 

 tant 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 a nearly north dip 

 at an angle of about fifteen degrees. Immediately above these and 

 without any discordance comes the first of the conglomerate beds, 

 which consists of a bed about ten feet thick of pebbles mingled with 

 slates. This is surmounted by about thirty feet of slates having a 

 most perfect cleavage in the plane of stratification. It is not difficult 

 to split a sheet a foot square having a thickness of not over one 

 twentieth of an inch. Immediately above this slate, which was evi- 

 dently deposited in deep water, for it is remarkably uniform in its 

 character, the conglomerate comes in again and continues with its 

 northern dip at an average angle of about fifteen degrees for a hori- 

 zontal distance of over a mile. Near the edge of the Charles River 

 alluvial flats, in Brighton, this conglomerate changes suddenly into a 

 sandstone which is throughout marked with very distinct ripple lines, 

 and bears other evidence of shallow water origin. Above this sand- 

 stone mass there comes a great expanse of amygdaloidal trap, which 

 at some points is seen cutting the sandstone and again appears to 

 have been poured out over its surface as a contemporaneous sheet. 

 As is often the case near great outbreaks of this kind, there are at 

 least two faults, one on either side of the dyke, which have produced 

 considerable dislocation in the beds. These are the only faults visible 

 in this most interesting section. 



Although it is by no means certain as yet, still it may be assumed 

 as probable that the Cambridge slates and Roxbury conglomerate 

 belong to the same great series of beds. The coincidence in the di- 

 rection of dip as well as the general character of the beds themselves 

 renders it probable that it may eventually be found that these slates 

 and conglomerates form part of the same series of beds as the Brain- 

 tree series, and all belong to the primordial era. I am assured by 



