251 . 



Sudbury River Aqueduct a purplish slate dipping N.W. per- 

 haps forty-five degrees. South of the slate there is room for 

 more conglomerate before the first of the amygdaloid is reached, 

 but I believe none is exposed. The aqueduct tunnel one- 

 fourth mile north of the Charles River, in the Village of South 

 Natick, is excavated in the rocks of this synclinal ; and they 

 are also exposed in natural ledges at either end. Two bands of 

 conglomerate enclose a band of slate, and all show a very high 

 north-west dip. The conglomerate is brownish and greenish, 

 small-pebbled, schistose, and largely composed of pinite. 

 The slate is of the same colors, soft and unaltered. On the 

 north the conglomerate is in contact with, apparently dipping 

 against, Huronian quartzite ; while on the south it appears to 

 overlie the slaty and felsitic rocks which I have referred to the 

 Shawmut group. This pinched up and overturned synclinal, 

 growing gradually narrower, but still including some slate, 

 and otherwise essentially unchanged lithologically, may be 

 traced south-west as far at least as the east end of the Rockland 

 Street tunnel of the aqueduct, one-fourth mile west of Cottage 

 Street. The slate at this point is reddish-brown, and divides 

 into small lenticular masses, with surfaces of chlorite. 



According to my observations, this is the extreme western 

 limit of the uncrystallines of the Boston basin ; and I think we 

 may safely conclude that they never extended much farther in 

 this direction. For the rocks of this long narrow syncline — 

 which has been traced fully six miles from the point where it 

 branches off from the great mass of the conglomerate and slate 

 in Newton, with a breadth probably not exceeding one- 

 half .mile, and gradually diminishing westward — are clearly an 

 estuary deposit, having been laid down in an elongated arm of 

 the sea. In other words, when, taking a general view, the 

 western shore of Boston Harbor was near the eastern boundary 

 of Needham, a contracted channel, analogous to the lower or 

 tide-water portion of the modern Charles, reached six miles 

 farther west. Of course folding and denudation have greatly 

 diminished the breadth of this belt of rocks, and yet that they 



