1875.] . 399 Podge. 



examined it carefully, approaching, in the vicinity of Wrentham, the 

 larger basin which lies in Bristol County. This may be called the 

 Norfolk County basin. 



To give a clear idea of the various kinds of rocks of the Boston 

 and South Shore basins, and their probable relations to each other, 

 and at the same time to avoid making the observed facts appear unre- 

 liable by being involved with the conclusions, perhaps mistaken, 

 drawn from these, the safest method will be perhaps to take up the 

 several kinds separately. 



A. Slates. 



In nearly, if not quite, all the places where these mixed rocks, such 

 as lie in the vicinity of Boston, occur, there is a mass of slates ap- 

 parently between the crystallines and conglomerates. Prof. Hitchcock 

 mentions them as found at Sachuest Point, Newport, at Portsmouth 

 in the north of the island, and at other places on it, at Little Comp- 

 ton, at Parker River, Newbury, Mass., and elsewhere. (Proc. Am. 

 Ass. Adv. Sci., xiv., 1861). So, too, these slates or novaculites are 

 found in places throughout the areas under consideration. They 

 probably underlie the whole district, but by far the greater number 

 of the outcrops group themselves in belts whose positions should be 

 noticed as throwing some light on the general arrangement and place 

 of the slates which compose them, in the whole system. 



(a.) On the north of the Boston basin these slates form a band 

 about three, or inside of three and a half miles wide, parallel to the 

 line of crystallines already described. They may be traced through 

 the western half of Newton among the conglomerate which is there 

 abundant, as far as the Charles River. Other slates which occur in 

 Newton are clearly a part of the conglomerate system. These often 

 resemble the Cambridge slates (to adopt the name given by Prof. 

 Shaler), but are usually thinner bedded and less hard. This band 

 includes Egg Rock, Nahant, Everett, Maiden, Somerville, Medford, 

 Cambridge, Belmont, Watertown, Brighton and Newton. 



The strike is E. N. E. - W. S. W. with N. N. W. dip, changing in 

 Newton to N. E. or N. N. E. strikes,with dip N. W. or W. N". W., 

 conforming to the bend of the line of enclosing crystallines. The 

 axis of elevation in Somerville and Cambridge is N. W. - S. E. as if 

 the strata were crimped up by the bending together of the ends of 

 the band. The strike is almost invariably parallel to Highland Street, 



