Benton.] 499, _[February 18, 
considerable angle with each other, and it is common for them to 
include triangular spaces between them. 
Many of these crystals show in polarized light that they are plag- 
ioclase. The rest have suffered chemical alterations into other min- 
erals, but they still retain their characteristic shape and relative 
position. In none of the sections examined, have they been entirely 
obliterated. Thus they are in all respects similar to the feldspar 
erystals, whieh are peculiar to basaltic rocks. Many of them show 
in polarized light that they have been partially or wholly changed to 
a monoclinic feldspar, which is probably orthoclase; others have 
been converted into a pale green substance, which is in some cases 
viridite, in others chlorite. The magnetite occurs in grains and fine 
dust, usually in connection with the’epidote. It is most abundant 
in the purplish part of the rock. 
The chlorite and viridite in many sections make up the mass of 
the rock. They are more abundant in the greenish part of the rock. 
The epidote occurs in irregular grains, disseminated through the 
groundmass, as also do the kaolin, and other earthy products of alter- 
ations. 
With the exception of the plagioclase feldspar, all the minerals 
which now constitute this rock are the products of chemical alter- 
ations which have taken place since the rock solidified. 
CONCLUSIONS. 
The shape of the plagioclase crystals, their arrangement, and the 
character of the alteration products which have just been described, 
stamp the amygdaloid as a melaphyre. As has been stated, its feld- 
spars are in every respect similar to those which are peculiar to all 
basaltic rocks, and its principal alteration products are similar to 
those found by Dr. Wadsworth in certain less altered melaphyres 
and diabases occurring in Boston, Somerville and Malden. Also to 
those observed by Prof. Raphael Pumpelly in the amygdaloidal mela- 
phyres of Lake Superior. 
Prof. Pumpelly,. Dr. Wadsworth, and Mr. S. Allport, have been 
able to trace the derivation of the alteration products from the orig- 
inal constituents of basalt. In the rocks now under consideration, it 
is, on account of the extreme alteration which they have undergone, 
impossible to do that, but we may safely infer that this rock was at 
the time of its solidification a basalt, and that its present variation 
from that rock is due wholly to subsequent changes. 
