262 



alteration consists in induration, much of the slate being of 

 flinty hardness, and in the development along certain lines of 

 large, elongated amygdules similar to those characterizing the 

 slate at Mill Cove, in Weymouth. The comparatively thin 

 layers possessing the amygdaloidal structure are mostly of a 

 yellowish or greenish-white color. The calcareous beds are 

 aggregated along two lines, near the middle of the slate ; con- 

 sisting of layers a few inches thick interstratified with the slate, 

 and not always clearly distinguishable from it ; they have a 

 volume, taken collectively, of perhaps twenty feet. The color 

 of the limestone is white to gray, and its texture varies from 

 compact to finely crystalline or saccharoidal. Thin seams 

 and strings of silicious material become evident on the weath- 

 ered surface. A specimen selected as probably representing 

 the purest of the limestone was analyzed by Miss Jennie M. 

 Arms , with the following result : — 



Insoluble residue (mostly Si 2 ) 29.6 



CaC0 3 69.1 



Mg C0 3 not determined 



98.7 



I have already called attention to the plain indication which 

 these calcareous beds in the eastern part of the Boston basin 

 afford of deepening water, in ancient as well as in modern 

 times, in this direction. It is instructive to compare these 

 deposits formed in the open sea with the highly ferruginous, 

 estuary strata in the vicinity of Charles River Village and 

 South Natick. 



The nature of the contact between the slate and the coarse 

 pyroxenic rock which meets it on the north-west is uncertain. 

 At several points the slate seems to dip beneath the crystal- 

 lines, — a circumstance which would favor the theory that the 

 slate is folded in a closed syncline with the axial plane inclined 

 to the north-west. But the entire absence of arenaceous and 

 conglomerate beds along the contact, as well as elsewhere in 

 this vicinity ; the undoubted exotic nature of most, at least, of 



