255 



exceeding thirty degrees. The slates on the north are, there- 

 fore, presumably the oldest and lowest. 



The Somerville slates are, on the whole, quite homogeneous ; 

 the texture is usually fine and even, and the principal colors are 

 gray, bluish-gray, and nearly black. The stratification is 

 marked in colors, and is ordinarily distinct, although the bands 

 are narrow and little conspicuous. Taking a general view, the 

 slate, as noted by Mr. Dodge, becomes gradually coarser and 

 more arenaceous toward the north. This is a significant fact, as 

 indicating a passage to conglomerate in that direction, or down- 

 wards, and thus showing a harmony between this and the other 

 slate areas in the Boston basin. Pyrite cubes are of common 

 occurrence in the slate, and at some points the slate is decidedly 

 pyritiferous. Wave-marks, contortions, minute faultings, and 

 anything approaching cleavage, are of rare occurrence. The 

 slate is frequently altered in the immediate vicinity of in- 

 trusives, but never to the extent observable in Brighton or 

 Nantasket. The steepest dips are on the north, and in this 

 direction particularly there are frequent and abrupt changes in 

 the direction and amount of the dip ; so that in a distance of a 

 few rods, or even yards, we may pass from horizontal strata to 

 those that are vertical. 



These slates are traversed by many dykes of diabase and, 

 possibly, other basic exotics. The dykes are so numerous 

 that nearly every quarry and outcrop show one or more, and 

 they are of all sizes, from mere threads to masses hundreds of 

 feet across ; while the coarseness of the material is, in general, 

 obviously proportional to the size of the mass. According to 

 their trends, the dykes may be divided roughly into two sets, — 

 one set coinciding in a general way with the strike of the slates, 

 and the other running. at right angles to this, or parallel with 

 the dip. Many of these exotic masses, including to some 

 extent the similar rocks of adjoining towns, have been studied 

 microscopically by Mr. M. E. Wadsworth,^ to whom we are 

 indebted for all that is definitely known concerning their composi- 



iProc. Bost. Soc. Nat. Hist., XIX., 223. 



