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enormous weight of the beds themselves, the arches not being 

 self-sustaining. I do not consider, however, that all the fract- 

 ures of the slate and conglomerate have originated in these 

 rocks, for in many cases they are probably but the upward 

 extension of dislocations which had their inception in the under- 

 lying crystallines. The very considerable narrowing of the 

 Boston basin, of which the folded slate and conglomerate afford 

 abundant evidence, must have necessitated a yielding of the 

 crystalline floor ; and there are many indications that this was 

 effected with the development of a minimum of plasticity ; 

 faulted folds being the rule, and igneous extrusions the com- 

 paratively unimportant exception. 



South Shore District. — At the southern end of Nantas- 

 ket Beach, in Hull, the rock lying farthest to the north, and 

 accessible only at low tide, is a very distinct conglomerate. It 

 holds many pebbles of greenish amygdaloid, including both the 

 slaty and typical varieties of that rock, and much of the paste 

 has evidently been derived from the same source ; there are 

 also numerous pebbles of granite 1 and petrosilex ; and I have 

 observed several pebbles unquestionably representing the Shaw- 

 mut breccia. The petrosilex pebbles embrace several varieties, 

 mostly reddish. On the north side of the ledge a layer of very 

 fine red sandstone or slate is intercalated with the conglom- 

 erate, while on the south side it is overlaid by a laminated 

 slate of a beautiful green color. These finer-grained rocks 

 show a southerly dip of 10°-20°. 



The conglomerate exhibits no special disturbance or altera- 

 tion, but the most casual examination of the slate shows that 

 it is greatly indurated, having about the hardness of ortho- 

 clase. A dyke of a black, crystalline, basic rock divides the 

 ledge in the direction of the dip, and is accompanied by a 

 slight dislocation of the beds. After three or four feet in 

 thickness of the green, novaculitic slate, the section is inter- 



1 Wherever, in these descriptions, granite pebbles are mentioned as occurring in the 

 conglomerate, the coarse, typical Huronian granite is to be understood, save where it 

 is otherwise stated. 



