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conglomerate, apparently a denuded anticlinal. This con- 

 glomerate is principally composed of the petrosilex and amyg- 

 daloid which it overlies. These rocks can be observed to the 

 best advantage in the area north of Pine Tree Brook, and east 

 of Blue Hill Avenue. 



North of this somewhat uncertain anticlinal, and south of the 

 conglomerate of Squantum and Dorchester, there is but one 

 exposure of slate. This, as already stated, is at the quarries 

 in North Quincy, near the harbor and north of Sachem's Creek. 

 It is a beautiful black and brown banded slate, elegantly jointed, 

 breaking into oblique rhombohedrons of small size. Strike, 

 E.N.E. ; dip, northerly, but nearly vertical. The bedding is 

 remarkably straight and regular, being free from contortions 

 and other inequalities. The nearest outcrops on this line of 

 strike, to the eastward, are on Quarantine Rocks and Rains- 

 ford Island, four miles down the harbor. Here, too, the rock 

 is all slate ; it is a homogeneous, grayish-black variety, much 

 like the slate of Raccoon Island, but finer, and differing from 

 that at the Sachem's Creek quarries chiefly in being only occa- 

 sionally banded with brown. But, though not essentially un- 

 like the slate of the locality last named, and having the same 

 strike, — N. 60°-70° E., — the conditions of its occurrence 

 are very different. On Rainsford Island the outcrops are con- 

 fined to the western half of the island, which is crescent-shaped, 

 two projecting points of land enclosing a semicircular bay. The 

 dip of the slates is everywhere very variable and undulating. 

 On the northern horn of the crescent it is sometimes to the 

 north, but will average to the south, or south-south-east, at a 

 low angle, probably 20°-30°. The strata are really considera- 

 bly contorted on a rather large scale, presenting in a small area 

 good examples of normal, overturn, and faulted folds. The 

 bedding is more or less obscured by the cleavage, which is quite 

 well developed in portions of the rock ; this is parallel with the 

 strike, and dips north, as usual, about 80°, entirely indepen- 

 dent of the stratification. On the south side of the bay, sub- 

 stantially the same phenomena are repeated, but the dip 



