142 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
black uncertain decomposition product. The fillings of the cavities 
evidently do not arise from segregation, as they are barely attached 
to the walls of the cavities, and it is almost impossible to grind a 
section of the rock without detaching them. They seem rather to 
result from mineralizers in the cavities of the rock. They vary from 
microscopic size up to nearly a centimeter in diameter and are scat¬ 
tered through the rock without particular arrangement. Various 
writers (Shaler, ’79, Benton, ’81, Davis, ’81, Wadsworth, ’83, 
p. 10, and Merrill, ’86, p. 439, and ’93) have described similar 
melaphyrs fro nr Brighton and Nantasket. 
DIKES. 
The dike rocks are for the most part diabases too much altered 
for satisfactory determination. Similar diabase dikes have been 
described from other portions of the Boston Basin (Merrill, ’93, 
p. 38-44), from many points along the coast of New Brunswick, 
(Matthew, ’95), and Maine (Shaler, ’89, and Kemp, ’90), about 
Cape Ann (Shaler, ’89), at Nahant (Lane, ’89), in Essex Co., 
Mass. (Sears, ’94, p. 130), in the great Somerville dike (Hobbs, 
’88), as well as along Lake Champlain (Kemp and Marsters, ’91 
and ’93, and White, ’94, p. 229). 
From the field relations Professor Crosby concludes that the dia¬ 
base dikes are of three sets in regard to age. On petrographic 
evidence, however, there is no means of separating them, all the 
more decomposed diabases appearing very nearly identical. 
For the purpose of proving the age distinction, if possible, a few 
typical dike rocks were collected whose relative age was evident in 
the field. In many slides the alteration is so complete that only a 
dusty aggregate of chloritic products, kaolinized feldspar, leucoxene, 
ferruginous discoloration, and some of the unaltered quartz is shown. 
They are soft when weathered, but very compact, of a dark green 
to black color. In many cases no particles of fresh augite remain, 
and the chloritic alteration product obscures everything. 
Dikes in the slate. Two dikes cut the Paradoxides slate in the 
Hayward Creek locality. They are of dark gray decomposed 
diabase, the augite mostly changed to feebly pleochroic chlorite,— 
the so-called “ viridite ” of various writers, — giving the rock a green 
color. The orthoclase is pinkish and shows traces of altered lath¬ 
shaped crystals. Pyrite is present in microscopical crystals. Much 
