VOLCANICS OF THE MICHIGAMME DISTRICT. 807 
oriented and sufficiently fresh sections upon which to make 
measurements an exceedingly tedious one. The rocks frequently 
show a flowage structure caused by the parallel arrangement of 
the microlites around the phenocrysts andamygdules. The min- 
erals of the andesite have suffered the same changes as those of 
the basalts. Andesitic fragments are also very common in the 
tuffs, although in no place do we have a true andesite tuff or 
breccia. 
As is seen from the description of the freshest and most 
characteristic of the basic volcanics, the mineral constituents 
have undergone very far reaching alterations, the result of which 
has been to produce most commonly epidote, zoisite, fibrous 
green hornblende, chlorite, and less quartz, feldspar and calcite. 
More advanced alteration results in rocks very similar to the 
basalts and andesites as described above, in which, however, the 
feldspar crystals are replaced by chlorite and epidote, the feldspar 
shapes being retained. In such cases the ophitic structure 
shows nicely in ordinary light, but between crossed nicols the 
feldspars break up into aggregates of chlorite and epidote, com- 
pletely concealing the characteristic structure of the rock. The 
very fine grained lavas undergo the same alteration, producing 
a very tough light green rock which does not show at all the 
structure of an eruptive rock, as they are frequently found free 
from amygdules. In the very fine grained rocks the production 
of so much epidote and titanite in minute grains, by their high 
single refraction, causes the rock to appear almost opaque in 
thin section. 
In addition to these changes, which have been for the most 
part merely the replacing of one mineral by another, the rocks 
have been subjected to considerable pressure and shearing, which 
have in many places given them a more or less perfect schis- 
tosity. In such cases the flattening of the amygdules is espe- 
cially noticeable. 
The amygdules are filled by nearly the same minerals as 
those which occur secondarily in the rock mass itself. Arranged 
in order of frequence of occurrence they are as follows: Epi- 
dote, zoisite, chlorite, quartz, calcite and feldspar. The non- 
