Crosby.] 



26 [January 16, 



to the north. The relations of the slate to the conglomerate on 

 the south are sufficiently clear at Newton Centre and Newton 

 Upper Falls. This belt of conglomerate becomes smaller-peb- 

 bled toward the north, and in the upper 300 feet includes several 

 beds from one to twenty feet thick of brown and gray sandstone 

 and slate ; thus passing gradually into the main belt of slate, 

 which it underlies conformably. The relations of the slate to 

 the conglomerate on the north have been observed at only two 

 points — Newton Centre and Chestnut Hill Resevoir. At the 

 latter locality, the conglomerate, like that south of the slate at 

 Newton Centre, is small-pebbled and includes several beds of 

 slate and sandstone — beds of passage between the two forma- 

 tions which are here also perfectly conformable. These facts 

 greatly strengthen the view that these two belts of conglomerate 

 are of the same age, and continuous under the slate, as they seem 

 to be around its western end. And, following this slate east- 

 ward, we come to the Providence Street well in which, as already 

 described, we see a great thickness of slate apparently resting on 

 conglomerate. The perfect cleavage parallel with the stratifica- 

 tion which the slate shows at most points is also favorable to the 

 view that it has been mashed up in a closed fold, experiencing in 

 this way much greater compression than that due to the mere 

 weight of the overlying beds. 



This view is consonant with all the facts so far referred to ; 

 and affords the only explanation of the stratigraphy that harmon- 

 izes well with the facts observed elsewhere in the Boston basin. 

 For, on the other hand, if the synclinal view is rejected, we at 

 once double the thickness of the entire formation, and have to 

 consider that the two great and distinct conglomerates, with the 

 slate between them, all dip to the north against the broad belt of 

 amygdaloid, which thus becomes a dike nearly a mile wide, in- 

 stead of what it more probably is, outflows and sheets of lava. 

 And where these sedimentary rocks come to the surface again it 

 is impossible to determine. Neither could we on this supposition 

 satisfactorily explain the fact that the slate between these two 

 belts of conglomerate varies in thickness from perhaps 150 feet 

 to at least 600 or 800 feet in a little more than a mile. But the 

 explanation of this variation is easy and natural, if we suppose a 

 synclinal deeper, and hence including more of the overlying 

 slates, at some points that at others. 



