Dodge.] 400 [February 3, 



which may serve as a convenient reference on a common map. There 

 are also indications of a parallel uplift along the line dividing Bel- 

 mont and Watertown, though the ridge here may have been formed 

 by exterior agencies. There may be some connection between this 

 axis, and the average N. W.-S. E. direction of the zigzag northern 

 side of the Dedham and Blue Hill crystalline area. 



The dips in Somerville 1 are very various, the rocks ranging from 

 nearly horizontal to vertical. On the southwest side of College 

 Hill, there is a high ledge dipping 89° S. W., while three quarters 

 of a mile from there, on lower ground, at the quarry on Elm Street 

 opposite the old powder house, low dips in almost all directions may 

 be found round the dyke which cuts the slate. The most frequent dip 

 is 10° to 20° S. W. Good exposures of the edges of the slates are on 

 the north side of Central Hill along the Lowell Railroad and the Lex- 

 ington Branch, northwest from Central Street (Winter -Hill Station), 

 and along the Medford turnpike. Perpendicular to the bedding, they 

 are quarried extensively near the Alms House, on Elm Street, and on 

 Milk near Lowell Street. 



The upturned edges are so cut by eruptives, and faulted, that it 

 would be impossible to determine the true thickness of the slates by 

 a section here. Considering the exposures as they lie, the strata to 

 the south-west are more uniform in character, being compact, homo- 

 geneous masses of felspathic rock, sometimes banded in gray and 

 purple ; while going toward the Mystic River they become more sandy. 

 At the Elm Street quarry, the strata are sometimes bands several 

 inches thick, as in the quarries along Milk Street, but again we find 

 close interstratification of slate and sandstone, and wave markings 

 appear at times. Along the Lexington Branch Railroad, the strata 

 are loose nodular grits as well as slates. Still farther north, the com- 

 pact varieties appear again and are quarried on Medford turnpike. 



Although there are a few scattered boulders of conglomerates in 

 Maiden and Somerville, the only ledge of conglomeritic rock which I 

 have seen in place north of Charles River, lies in Watertown between 

 School, Belmont, Arlington and Mt. Auburn Streets. It is a passage 

 from pudding-stone to slaty or schistose rock north-westerly. The 

 projecting quartz pebbles at its southeast end show a clean joint at 

 that place, although the softer rock has worn away about them. 



1 Somerville was apart of Charlestown when the State Geological Report was 

 published (1840). 



