249 



situ by the decomposition and exfoliation of the irregularly 

 cuboidal blocks into which the rock joints. 



East of Allston Street the amygdaloid on this line appears 

 to divide so as to include a large mass of conglomerate, with 

 sandy layers, and the usual northerly dip. Directly north of 

 the Water- Works, and midway between Cambridge and North 

 Beacon Streets, a thin-bedded, undulating, gray slate dips N. 

 20° E. 32° ; and this is followed on the north side of North 

 Beacon Street, a few rods west of Everett Street, by con- 

 glomerate with sandy and slaty beds dipping N. 10°— 15° 

 E. 30°-35°. Along certain lines, especially, the slaty layers, 

 as already observed [ante, p. 190), are small and lenticular, 

 often resembling pebbles. In this ledge, which is on the 

 edge of the Charles River alluvial flats, and the last exposure 

 south of the river in this part of its course, the finer materials 

 are most abundant toward the north, indicating a passage to 

 slate in that direction ; and since we are now beyond the 

 range of the amygdaloid, and fewer repetitions of the strata 

 are to be expected, I think it is safe to conclude that the flats 

 are underlaid chiefly, if not wholly, by slate. This conclusion, 

 as will presently appear, is confirmed by observations made 

 farther west. 



This band of conglomerate bordering the amygdaloid on the 

 north is represented by few and widely separated exposures in 

 its western extension. It shows everywhere a northerly dip, 

 and generally at high angles, especially along the northern 

 edge, where, as just hinted, there are plain indications of a 

 passage to slate. The relations of the slate and conglomerate 

 on this line appear to be essentially the same as along the 

 northern border of the West Koxbury and Dorchester slate 

 belt. 



At the corner of Lake and Washington Streets, sandstone 

 and conglomerate, closely involved with the amygdaloid, dip 

 gently to the north. A short distance east and south-east of 

 the station at Newton Corner, on and near the railroad, there 

 is an unquestionable gradual transition from conglomerate to 



