1877.] 223 [Wadsworth. 



THE INTRUSIVE ROCKS IN THE VICINITY OF BOSTON, 



It is proposed in this paper to discuss only part of the intrusive 

 rocks to be found within the limits of Somerville, Maiden, Medford, 

 Brighton, Cambridge, Brookline, and Roxbury, and bounded princi- 

 pally by the porphyritic felsites and the medial line of the con- 

 glomerate. 



The country rock in which these intrusives occur is principally an 

 argillite, with varying dip, and a conglomerate; in this there occur 

 five apparently different varieties of intrusive rocks. 



In order to separate my views and theories from the facts ob- 

 served, I shall, when possible without too great violence to the gen- 

 eral appearance of these rocks, give them for the present the names 

 by which they have been known heretofore, reserving my deductions 

 for a later portion of this paper. 



The first class is typically represented at the prehnite locality of 

 Somerville, and is ordinarily known as "granite," "syenite," or 

 "diorite." 



The second class occurs in narrow dikes, which President Hitch- 

 cock calls " trap " in his Reports. 



The third class is composed of the well known " greenstones " of 

 this vicinity. 



The fourth class is typically represented on Fremont Street, Som- 

 erville, and has been called "granite" and "limestone," but for the 

 present it may be well to call it " porphyry." 



The fifth class is the so-called " amygdaloid " of Brighton. It is 

 not intended to discuss the amygdaloid in this paper, but to confine 

 it to the first four varieties named. 



I shall first give a list of localities, and the rocks to be found in 

 them, and afterwards describe part of them more in detail. 



LOCALITIES IN SOMERVILLE. 



1. Head of Granite Street, off Somerville Avenue. The rock 

 here is the " diorite " of the prehnite locality. 



2. Corner of Somerville Avenue and Lowell St., — "greenstone." 



3. On Lowell Street, midway between Somerville Avenue and 

 Summer Street, — " greenstone." 



4. In the triangle formed by Broadway, Holland, and James 

 Streets, — " greenstone." 



5. Newbury Street, near Broadway, — " greenstone." 



PROCEEDINGS B. S. N. H, — VOL. XIX. 15 NOVEMBER, 1877. 



