MUSEUM OF COMPAKATIYE ZOOLOGY. 3 



No outcrops of the coarse" rock have been found south of the Granite 

 Street quarry. Professor Crosby has included the fine-grained diabase 

 "which crops out at the Pumping Station in Brighton, and similar rocks 

 in Brookline and Xewton, as a part of this dike ; but the great change 

 of strike required, and the long intervening distance without exposures, 

 are opposed to the supposition. From the Granite Street quarries to 

 the Old Powder House in Somerville, (a distance of about one and a half 

 miles,) the strike of the exposures is N. 25° "NV. From that point to 

 Spot Pond, the trend is N. 10° E. In Medford and Somerville the 

 country rock is argillite, Avhich has been thrown into gentle folds, the 

 dips of which seldom exceed 35°. A notable exception to this state- 

 ment is seen at the old slate quarry on Professor's Row, College Hill, 

 where beds strike ±N. 95° E. and dip ±72° to the south. The area 

 of coarse diabase, which has an average width of about two thousand feet, 

 is never found in contact with the slate. The exposures of diabase al- 

 most invariably show the well-known weathering to boulders in situ, 

 though this is best observed at Pine Hill. North of High Street in 

 Medford the areal geology is complicated by the occurrence of granite 

 and felsite, for the mapping of which very detailed field-work v^'ill be 

 necessary. 



The arrangement of exposures of coarse and fine grained diabase in 

 the vicinity of the Old Powder House seems to show a gradual passing 

 of one rock into the other. In the immediate vicinity of the Powder 

 House is an extensive outcrop of coarse rock, like that at the Granite 

 Street quarries and Pine Hill. About four hundred feet northeast of the 

 Powder House on Harvard Street the texture is much finer, though not 

 sufficiently fine to be ranked with the normal " gi-eenstone." About six 

 hundred feet S. 20° W. of the Powder House the rock is somewhat finer 

 than at the last-mentioned locality. Again, at the corner of Elm and 

 Morrison Streets, which is about one thousand feet west-southwest of the 

 Powder House, the normal " greenstone " occurs in slate. Moreover, on 

 Willow Avenue, about fifteen hundred feet along the strike to the south 

 from the Harvard Street locality, the rock is practically identical with that 

 at the latter place. From this it seems probable that the coarsely crystal- 

 line rock at the Powder House is near the middle of the dike, where the 

 cooling was slow, and that the gradual diminution in the size of the 

 grains in going from that point is owing to more rapid cooling near 

 the contact. 



The wide distribution of the " greenstone " has made it impracticable 

 for me to make a complete examination of it, but the few localities which 



