253 



Railroad and theCrystallhies of Waltham, A.rlington, Med- 

 ford and Maiden. — Between the Boston and Albany Railroad 

 and the Charles River on the north I know but one exposure of 

 the rocks, and this I have not seen. It occurs in Morse's Field, 

 near Newton Corner, and is described by Mr. Dodge^ as varying 

 from pure slate to fine sandstone, with a dip N. 20*^ W. 50°. 

 On the north side of the river, according to the same observer, 

 there is slate in the cattle market. The first rock exposed 

 north of this is in the square bounded by Mt. Auburn, School, 

 Belmont, and Grove Streets. Here is found the only ledge of 

 conglomerate known between the Charles and Mystic Rivers. 

 The rock is small-pebbled, slaty, and evidently composed mainly 

 of the debris of amygdaloid. The dip is not observable in the 

 conglomerate, but toward the south this rock is clearly traceable 

 into a sandy slate with a moderate southerly dip ; and this 

 changes on the south, again, as is indicated by a ledge a few 

 rods south-east of the conglomerate, to a fine, gray slate, dip- 

 ping S. 20° E. 70°-80°. The material evidently becomes finer, 

 and the dip steeper, away from the conglomerate. The im- 

 perfect cleavage of this slate dips north in the neighborhood of 

 seventy degrees. There is evidently exposed here only the 

 upper part of the conglomerate formation, where it is changing 

 to slate ; and its position is undoubtedly anticlinal. The slate 

 between this point and the conglomerate south of the river, 

 it will be observed, shows synclinal dips ; but the breadth of 

 this belt, together with the high average dip, leads to the in- 

 ference that there is probably an anticlinal entirely concealed 

 here. Near the junction of Common and Orchard Streets, in 

 Watertown, there is enough conglomerate in the drift to war- 

 rant the belief that this rock actually occurs here in situ, 

 though not exposed. It is of the same general character as 

 that between School and Grove Streets, and is on precisely the 

 same line of strike, though nearly a mile distant ; this is a pretty 

 strong indication that the conglomerate anticline is continued 

 in this direction. 



•Proc. Bost. Soc. Nat. Hist., xvii., 401. 



