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No. 2.— The Structural Relations of the Amygdaloidal Melaphyr 
in Brookline, Newton, and Brighton, Mass. By Henry T. 
BurRR. 
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 and 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 no means of dis- 
tinguishing them, petrographically, from the melaphyr. 
Macroscopically, the extent 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 
