1884.] 11 [Crosby. 



these are the only points in the Boston basin where limestone is 

 known to occur. The occurrence of fragments of Paradoxides 

 in the drift of George's Island, six miles north-northeast from 

 Hayward's Creek, is a strong indication that the slates are of the 

 same age in distinct portions of the basin. The slate on the 

 extreme northern edge of the basin, in Maiden, is indistinguisha- 

 ble from that in the Paradoxides quarry. And much of the slate 

 in Somerville and in the West Roxbury and Dorchester belt is 

 very similar to that in the South Shore district. 



Such differences as are noticed in these rocks may often be 

 traced to peculiarities in the local conditions at the time the rocks 

 were formed. Thus, although the conglomerate is made up of 

 pebbles of about all the crystalline rocks of this region ; yet its 

 composition is usually influenced by the character of the adjacent 

 crystallines; a conglomerate adjoining felsite being largely com- 

 posed of felsitic debris, while one adjoining granite is iikely to be 

 full of granite pebbles, and the proximity to amygdaloid usually 

 insures the presence of numerous pebbles of this rock in the con- 

 glomerate. I will not now advert to the evidence proving that 

 during the deposition of the conglomerate and slate the Boston 

 basin had approximately the form now indicated by the bound- 

 aries of these rocks, being merely a westward extension of the 

 modern Boston Harbor, with, of course, a greater width. But I 

 am satisfied that this view is- correct; and it assists us in explain- 

 ing the character of the sediments in the different portions of 

 the basin. For instance, the only two localities where limestone 

 is known to occur — Nahant and Weymouth — are in the most 

 seaward portion of the basin and far from the old shore-line, the 

 source of the sediments. And it is in this direction, in Nahant, 

 the harbor islands, Somerville, Cambridge, the eastern part of 

 Boston and Dorchester, and the South Shore district, that the 

 slate is finest and most homogeneous. On the other hand, we find 

 in the more remote portions of the basin, as in Needham and 

 Natick, only such sediments as would be likely to be formed in 

 narrow and quiet arms of the basin well cut off from the main 

 body. The conglomerates here aie fine grained and slaty, being 

 generally composed of soft materials, although hard rocks are 

 abundant among the adjacent crystallines, and the slates are 

 large of a brown or red color, showing an abundance of ferric 



