MAR 21 1901 



No. 2. — TJie Structural Relations of the Amygdaloidal Melaphyr 

 in Brookline, Newton, and Brighton, Mass. By Henry T. 

 Burr. 



The sedimentary rocks of Boston and vicinity are associated with 

 basic igneous rocks of various ages. Local geologists are accustomed to 

 divide these, roughly, into two broad groups, — the trap dikes and the 

 "melaphyrs" or " amygdaloids." The latter are the greater masses, 

 irregular in outline aud much altered in structure and texture. The 

 former are distinctly dikes. They still retain much of their original 

 crystalline texture and are unaffected by cleavage. 



While there is seldom any difficulty in distinguishing the two types 

 in the field, it is not easy to define the points of difference. Under the 

 microscope the melaphyr is excessively decomposed. It has been 

 studied in the Brighton area by E. R. Benton ('80, p. 416), at Hough's 

 Neck, Quincy, by J. E. Wolff ('82, p. 232), and by T. G. White ('97, p. 

 140). These writers agree in describing the rock as an altered basalt. 

 It is made up, typically, of plagioclase feldspar, magnetite, epidote, and 

 a mass of calcite, chlorite and other more or less indeterminate alteration 

 products. I have nothing to add to this save that slides from various 

 portions of Newton and Brookline exhibit the same general characters. 

 The traps are likewise much decomposed, although in varying degrees. 

 Dr. T. A. Jaggar, Jr., in investigations not yet published, has found that 

 many of these are altered basalts. There seems to be uo means of dis- 

 tinguishing them, petrographically, from the melaphyr. 



Microscopically, the exteut of alteration in the melaphyr is usually 

 sufficient to obliterate the crystalline texture. The traps almost invari- 

 ably appear, to the eye, crystalline. The melaphyr has generally 

 assumed a greenish to purplish tinge, difficult to describe, but quite char- 

 acteristic. The melaphyr is frequently amygdaloidal. The amyg- 

 dules have a peculiar habit of grouping themselves about centres. 

 They are seldom scattered uniformly through any considerable mass 

 of the rock. The traps are seldom amygdaloidal, and the amygdules, 

 where they occur, tend to be rather sparsely disseminated. 



VOL. XXXVIII. — no. 2 



