WHITE : PETROGRAPHY OF THE BOSTON BASIN. 
141 
coating the walls of the amygdule. The amygdules in the surface 
of the flow are much more abundant than in the base, and the lath- 
shaped feldspars in the ground mass are much larger. Other 
minerals of the ground mass are magnetite, chlorite, and epidote ; 
with red hematite grains, kaolin, and other earthy decomposition 
products. The lath-shaped feldspars often show a tendency to 
flow texture. 
The melaphyr at the top of the flow, along the irregular contact, 
is of a slaty character, both the amygdules and the microscopic 
texture of the base showing the effects of a shearing action in many 
cases, while the larger feldspars show bendings. 
In the purple portion the olivine has almost entirely been replaced 
by a green product, but the olivine appears in the fresher greenish 
gray base of the flow, as shattered grains, with dark rims and 
brilliant polarization. Doubtless some of the apparent amygdules 
of fibrous chlorite and serpentine are pseudomorphs after olivine, in 
the process of alteration. The interior of one of these shattered masses 
also shows a gray tint, polarizing brilliantly, and probably, as 
Professor Wolff has suggested, indicating an alteration to talc. 
Feldspar in lath-shaped crystals is scattered through the sections 
(PI. 5 , Fig. 15). Neighboring crystals forming an angle with each 
other often have the included space filled by augite, which has 
generally altered to a green or greenish brown chloritic substance, 
sometimes traversed by red stains along the cracks. The feldspar 
was doubtless originally plagioclase, but its form is often entirely 
obliterated, or else it is changed to a monoclinic feldspar, probably 
orthoclase. Others are converted into pale green chlorite. Mag¬ 
netite is in grains like fine dust, usually in connection with the 
epidote. It is most abundant in the purplish portion of the rock. 
Apatite is of frequent occurrence. Epidote is in irregular grains 
disseminated through the ground mass, especially in the greenish 
portion of the rock. Microscopic veins of epidote also traverse the 
rock. 
At the base of the flow the rock almost wholly lacks amygdules 
and seems to be a true basalt. The base of the flow strongly 
resembles the melaphyrs of the hanging wall of the copper mines in 
the upper Michigan peninsula described by Irving (’83, p. 62.) 1 It 
contains none of the calcite or feldspathic amygdules, but shows 
large round amygdules nearly 1 cm. in diameter, of a brownish 
1 Compare Pumpelly (’80, p. 31); also on metasomatic development, p. 274, etc. 
