252 



were formed under the conditions named above is indicated by 

 their lithologic characters. The comparative tranquillity of the 

 water is proved by the fineness of the material deposited. The 

 conglomerate is small-pebbled and slaty, consisting mainly of 

 pinite and the debris of argillaceous amygdaloid, with very 

 few pebbles of harder rocks ; while the ferruginous char- 

 acter of both the conglomerate and slate, especially toward 

 the west, is a strong indication that the water was of limited 

 extent, and communicated but imperfectly with the external 

 ocean. 



The tongue of amygdaloid projecting east between the gran- 

 ite of Dover and the petrosilex of Needham points to an exten- 

 sion of this primordial estuary in that direction, the main trough 

 probably giving off a branch about midway of its length ; and 

 this surmise is confirmed bv the occurrence north and east of 

 Charles River Village of a band of conglomerate and sandstone 

 splitting the amygdaloid. Toward the west this conglomerate is 

 concealed by the drift, and its connection with the Newton and 

 Natick band cannot now be proved ; and yet there can be little 

 doubt that this was its original, if it is not its present relation. 

 As usual, under like circumstances, the conglomerate is closely 

 involved with the amygdaloid upon which it rests and of the 

 debris of which it is largely composed. The pebbles, except at 

 a few points where the rock is very coarse, are small ; and sandy 

 and even slaty beds are frequently included. Much of the rock 

 is of a deep red color, being highly ferruginous. This variety 

 is well exposed in the cuts on the New York and New England 

 Railroad, Woonsocket Division. At no other point in this 

 region is the ferruginous character of the conglomerate and 

 slate so strongly marked as here ; and it is highly instructive to 

 observe that this is the portion of the Boston basin which, dur- 

 ing primordial times, was most completely landlocked and most 

 remote from the open sea The color of these beds is an indi- 

 cation at once of their estuary origin and of their narrow limits 

 in ancient as well as modern times. 



The broad Slate Belt, between the Boston and Albany 



