232 



The relations of the conglomerate to the amygdaloid in New- 

 ton and Brookline are very puzzling at some points, indicating 

 that here, as in the South Shore district and Milton, the 

 amygdaloid has been, locally, plastic and intrusive. The occur- 

 rence of the Shawmut breccia here (see ante, pp. 171 and 173) 

 also increases the confusion, and adds to the difficulty of tracing 

 the exact limits of the conglomerate. The conglomerate in 

 the vicinity of Holyrood Cemetery, and generally around the 

 borders of the amygdaloid, is unusually coarse. The isolated 

 patches, veritable islands, of conglomerate lying on the amyg- 

 daloid are of especial interest, as showing the former continuity 

 of the Primordial beds over tliis denuded crystalline axis, and 

 proving the inferior position of the conglomerate with respect 

 to the slates. These islands seem to be numerous, but I have 

 only shown on the map the one first observed, and probably the 

 largest. The isolated masses of amygdaloid in the conglom- 

 erate between the main mass of the amygdaloid and the slates 

 on the north, which are likewise much more numerous than 

 represented, appear to be narrow, elongated, dyke-like masses, 

 conforming with the strike of the conglomerate ; but they are 

 probably to be explained partly by faulting, and not wholly by 

 extravasation, though there can be no question that some of the 

 material is truly exotic. 



It is, of course, not improbable that such a broad, unsym- 

 metrical fold should be somewhat broken. Evidence of one or 

 more faults along the southern border has been already pointed 

 out, and others doubtless exist ; but the rock is too homogene- 

 ous for their detection. The strongest argument for their occur- 

 rence is the consideration that, otherwise, even if we assume 

 the average dip to be as low as fifteen degrees, and it certainly 

 exceeds this, the volume of the conglomerate would be so great 

 as to be entirely inconsistent with the sections measured else- 

 Avhere. An average dip of fifteen degrees would, if the section 

 were unfaulted, correspond to a maximum thickness of nearly 

 two thousand feet. From its southern border northward, the 

 conglomerate maintains a high southerly dip, 60°— 90°, for 



