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rupted by a similar but larger dyke (twenty to thirty feet 

 wide), parallel with the strike. Here, too, there is a fault,, 

 with the downthrow on the north ; for the rock bounding the 

 dyke on the south is not slate or conglomerate, but a massive, 

 almost crystalline variety of amygdaloid. This has decidedly 

 the aspect of an eruptive, although easily distinguished from 

 the dyke rock. It is overlain a few feet farther south by a 

 greenish and yellowish sandstone, which has suffered an extraor- 

 dinary degree of alteration. The bedding is, for the most 

 part, entirely destroyed ; and where this structure yet remains 

 it is greatly disturbed ; but it can still be seen that the general 

 dip is S., about 10°. This rock is thoroughly indurated, and 

 holds numerous irregular masses of endogenous quartz. The 

 contact with the amygdaloid is well exposed ; and it is of such 

 nature as to show that, although apparently coinciding with the 

 bedding, it may be due to the extravasation of the last-named 

 rock. The alteration of this sandrock is without a parallel 

 elsewhere among the Primordial rocks of the Boston basin. 

 So great is the change that most observers, I believe, have 

 failed to distinguish the sandstone from the amygdaloid, through 

 contact with which it has experienced its metamorphism. The 

 derivation of the sandstone from the amygdaloid is evident from 

 its color. 



A few rods to the westward, just at the end of the beach 

 and the foot of the hill, are several good ledges of a similar 

 but less altered greenish sandstone, interstratined with beds of 

 rather an obscure conglomerate. The latter rock is largely 

 composed, both pebbles and paste, of amygdaloid. The dip, 

 as before, is S., 10°-20° ; but the beds are cut off in that 

 direction by the base of the hill, which is a great mass of 

 amygdaloid. There is a very obvious strike fault here, but the 

 contact is quite complicated, the amygdaloid having played to 

 some extent the role of an eruptive ; and the two formations 

 are, in consequence, difficult to separate. These stratified beds 

 are undoubtedly the same, or nearly so, as the highly altered 

 mass on the east, but a transverse fault evidently intervenes. 



