256 



tion and lithologic relations. He finds, as a general result, that 

 the present diversity of composition, and, in some cases, of text- 

 ure, is due to- subaerial decomposition; the original essential 

 constituents of all the rocks examined having been augite, feld- 

 spar, and magnetite, agreeing with diabase among the older 

 exotics, or dolerite and basalt among the newer. The coarsest 

 texture is found in the large quarry at the head of Granite 

 Street, off Somerville Avenue (the well-known prehnite local- 

 ity), arid at the Powder House, near the corner of Elm Street 

 and Broadway. These are both large masses : the latter has a 

 trend about IST. 20° E., and can be traced for one-fourth of a 

 mile ; but the boundary of the former cannot be seen at any 

 point, and the shape of the mass is unknown. By reference to 

 the map, it may be observed that these are nearly south of, and 

 approximately on a line with, the immense dyke in Medford, 

 with which I have already {ante^ p. 22) connected them, this 

 correlation being sustained by the observations of Mr. Wads- 

 worth ; and toward the south we find on the same south-by-west 

 line the dyke near the Brighton Water- Works, which, though of 

 finer texture, shows the same concentric decomposition as the 

 rock in Somerville and Medford ; while still beyond are the very 

 large dykes in the west part of Brookline and on Lagrange 

 Street, in Newton. This line is not less than ten miles long. 



At least two of the dykes cutting the Somerville slate in the 

 direction of the strike are traceable with considerable certainty 

 for long distances. One of these first appears on the west in the 

 large quarry on Tannery Lane, near the Cambridge Almshouse, 

 where it is about twenty feet wide, and nearly vertical, with a 

 trend S. 70° E. It crops at several points before reaching 

 Holland Street, and in the vicinity of that street it is split up so 

 as to reach the surface along several parallel lines. A few rods 

 north of the Powder House, where a narrow excavation exists 

 in the hillside, the large Powder-House dyke appears to be cut 

 by a dyke of finer texture. This is precisely on the line of the 

 Almshouse dyke ; and, continuing in this direction, a large 

 dyke forty to fifty feet wide, and dipping to the north nearly 



