Crosby.] 12 [January 16, 



oxide, which is common with estuary deposits, but rare in those 

 deposited in the open sea. The red color and flinty hardness of 

 some of the slates, and of some of the arenaceous beds in the 

 conglomerate, are due to the baking effect of the adjacent erup- 

 tive masses. 



The conglomerate and slate occur principally in large masses, 

 each rock covering tracts some of which are ten to twenty 

 square miles in extent. All the areas of these rocks are elongated 

 in an east-west direction, parallel with the general line of strike 

 and the axis of the basin ; so that a north-south traverse shows 

 several alterations of belts of conglomerate and slate. The cor- 

 rect and, I believe, generally accepted explanation of this feature 

 of the basin is that it indicates a series of anticlinal and synclinal 

 folds, the crests of the latter having been eroded so as to expose 

 the underlying rock. As I shall show later, the conglomerate is 

 the underlying rock, and hence its position is anticlinal and that 

 of the slate is synclinal. This relation is clear enough in most 

 cases, but the narrow band of slate running through Brookline 

 and the southern part of Newton is involved in an inverted fold 

 so that the conglomerate above the slate appears to be distinct 

 from that below it. This view is further strengthened by the 

 appearance of unconformability between the slate and overlying 

 conglomerate. Dr. Wadsworth has recently attempted to show 

 that these facts prove the existence of a formation of conglome- 

 rate newer than the slate. But I think it will appear later that 

 the interpretation of .the facts proposed by me is more natural 

 and satisfactory. 



The general separateness of the conglomerate and slate is an 

 important and well established fact ; for, although beds of fine 

 materials, sandstone and even slate, are of common occurrence in 

 the conglomerate, yet they are essentially a part of that rock are 

 always of limited extent, and clearly to be distinguished from 

 the great mass of the slate. Along the contact between the con- 

 glomerate and slate, especially, there are frequently several alter- 

 ations of beds of conglomerate, sandstone and slate ; but they 

 are clearly beds of passage and do not invalidate my general con- 

 clusion that there is, taking a general view, but one conglomer- 

 ate and one slate. 



3. The continuous and conformable deposition of the conglom 



