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this belt, underlying Boston and the harbor, if unfaulted, must 

 include several distinct folds. As already stated, the failure 

 of the slates to appear on the surface between the reservoir and 

 Newton Centre can scarcely be attributed to a scarcity of out- 

 crops, for these are abundant. There are three other hypothe- 

 ses which suggest themselves, but neither of them affords an 

 explanation which is entirely satisfactory. Firstly, we may 

 suppose that in this part of the syncline the denuding agents 

 have cut below the bottom of the slate. But this is improba- 

 ble, because the ground at this point is higher than to the east 

 or west, where the slates are now exposed ; and then it is 

 unlikely, too, that the not less than eight hundred feet, traverse 

 measure, of slate underlying the reservoir would taper out to 

 nothing, either vertically or horizontally, in so short a distance. 

 Secondly, it is possible that the slate is concealed by a strike- 

 fault, having the downthrow on the north. This might be an 

 approximately vertical fracture ; or, thirdly, we may suppose 

 that the inverted anticline represented by the conglomerate on 

 the north of the slate has been broken along the crest at some 

 points, and the upper half forced, by the horizontal pressure 

 producing the fold, beyond the lower half, cutting the slate off 

 in whole or in part. The last seems, on the whole, the most 

 satisfactory view ; for that this horizontal pressure has been 

 exerted, and that it has produced fractures in that direction, to 

 a' certain extent at least, we have evidence in the section rep- 

 resented by PI. 5, fig. 2 ; which may, I think, be regarded as 

 an example on a small scale, affecting a single layer of slate, 

 of the fracture and dislocation which have at certain points 

 concealed the whole breadth of that rock. 



I would suggest, too, that we may find in the pressure of 

 this overturned anticline, which originally embraced twice the 

 entire volume of both the conglomerate and slate and was 

 forced by tangential pressure as well as gravity against the slate 

 folded in the syncline, an adequate explanation of the eminent 

 cleavage parallel with the bedding — lamination-cleavage — 

 which forms the leading characteristic of the slate in this belt. 



