Dodge.] 402 [February 3, 



and Blue Hill Avenue cross; also east of Dorchester Avenue and 

 south, of Centre Street. The middle one of the three eastern points 

 of Squantum is formed by slates. Running across the harbor in the 

 same general direction, we come upon Rainsford and George's Islands, 

 Boston Light and Shag Rocks ; a succession of rocky islands forming 

 a long ledge about E. 26° N. Parallel to this lie Lovell's and Gal- 

 lop's Islands, which are based on rock, Middle and Outer Brewsters, 

 Martin's Ledge, etc. Some of these islands, which I have not exam- 

 ined, may consist entirely (as most which I have seen do partly), of 

 eruptive diorite and porphyry. The dip, as on land, is high, tending 

 to northerly when not vertical. 



(c.) In North Quincy near the harbor and north of Sachem's 

 Creek, there is a quarry of slates which may belong to the same 

 group. The strike is N. 55° E, and the dip varies from 3° to 6° from 

 vertical. The slate is pure, of a grayish color and conchoidal frac- 

 ture, and joints into oblique parallelograms of small size. 



Dr. Hitchcock speaks of slate on the south side of Hull, and there 

 are specimens of " Graywacke slate, Hull, " in the rock collections 

 in Boylston Hall at Cambridge, but I do not know their locality, and 

 have failed to find slate in Hull. 



(d.) At Hyde Park there are conglomerates and slates ; the latter 

 on Gordon Avenue opposite Austin Street, and at the corner of 

 River and Summer Streets. I have not ascertained whether the two 

 kinds are of the same age. 



: {e.) In Quincy, along Black's Creek north of Adams Street, there 

 •are conglomerates and slates very near the crystallines. The road 

 runs parallel to the northern limit of the latter for quarter of a mile 

 west of the Old Colony Railroad. Between the road here and the 

 crystallines, and also across the road where it turns southward, is a 

 hard black rock, possibly eruptive (as is often the case between the 

 crystallines and more recent strata), partly, perhaps, quartzite or fel- 

 site or very hard sandstone. Then just north of the brook, are large 

 grained conglomerates about 140 feet thick, and above these, after an 

 interval of 92 feet, hidden, are sandstones, 64 feet, and slates, 132 

 feet, nearly vertical and with strike N. 65° E., apparently the top of 

 the section, and glaciated. Along the railroad track, there are sand- 

 stone at the south, just north of the brook, and hard black slate with 

 strata marked in colors, but often flinty, resembling the slate of the 

 Braintree trilobite quarry. 



