265 



been since filled and re-excavated. It is scarcely necessary to 

 point out the support which the fact here established lends to 

 the doctrine of the comparative permanence of those topograph- 

 ical features fashioned from the crystalline formations. 



Volume of the Conglomerate and Slate. — The volumes 

 of both the slate and conglomerate vary greatly in differeut 

 parts of the Boston basin ; but I am satisfied that they nowhere 

 reach the enormous thickness which some writers have assigned 

 them. In the Roxbury and Brookline anticlinal, for example, 

 the conglomerate has an apparent thickness of four or five 

 thousand feet ; and the slates of Somerville and Cambridge, 

 taken as they lie, will scarcely measure less. But, since these 

 results are entirely inconsistent with sections in the adjacent 

 parts of the basin, where we have every reason to believe, not 

 necessarily the maximum, but the entire, thickness of each 

 rock is involved ; and since, as I have shown in the preceding 

 descriptions, proof is not entirely wanting that the broad folds 

 mentioned are extensively faulted in a way to augment the 

 apparent thickness of the strata, it is impossible to regard 

 these large numbers as even approximations to the truth. At 

 many points the structure, according to the interpretation 

 proposed in the preceding pages, not only does not require, but 

 will not allow, more than five hundred feet of conglomerate ; and 

 in some cases considerably less. Hence, when we pass almost 

 abruptly from such a section to one where the same rock has 

 an apparent volume ten times as great, it seems proper to seek 

 for some other explanation than a natural thickening of the 

 beds to that extent ; and this is found in the plain indications 

 of strike faults. 



Several observers, overlooking, as it seems to me, the 

 marked structural similarity of the conglomerate and slate, 

 have assigned the former terrane a vastly greater thickness than 

 the latter. Thus, Prof. N. S. Shaler, in speaking 1 of the con- 

 glomerate, especially that enclosing the Newton Upper Falls 



i Proc. Bost. Soc. Nat. Hist., xin., 175. 



