MUSEUM OF COMPARATIVE ZOOLOGY. 283 
in some of the basic gabbros. No inference, it seems to me, can be 
drawn regarding the heat to which a liquid has been exposed from 
either the pyrognostic or thermo-optical characters of the minerals that 
crystallize out on its cooling, or from the order of this crystallization. 
Thermo-optic and pyrognostic observations will prove to be of value, 
doubtless, when applied to those minerals that belong to the first class 
in volcanic rocks. 
As far as my own observation has gone, and as far also as can be told 
from the observation of others, no sulphide, no carbonate, nor any 
hydrous mineral whatsoever, is the direct product of crystallization of 
the fluid magma, but all these are the products of alteration and infil- 
tration after the consolidation of the lava, as are also part of the feldspar 
and quartz, as well as all of the epidote, ete. 
Cases of envelopment are of constant occurrence among the minerals 
of the second class, but they bear no resemblance to those cases of 
alteration that Dr. Hunt has endeavored to place under the head of 
envelopment, and which are so abundantly seen in the altered rocks, 
The alterations that take place are not usually attributable to ex- 
traneous material brought in by infiltrating waters, but are rather 
molecular changes brought about by the percolating waters within the 
rock mass. Except, therefore, on the weathered portions or in some 
rocks exposed to abnormal conditions, while the mineral constituents 
change through chemical rearrangement, the ultimate chemical consti- 
tution of the rock, as a whole, remains nearly constant. 
The microscopic and chemical characters of volcanic rocks are opposed 
to the idea that they came from sedimentary ones, as the inclusions of 
the first division of the first class are nearly constant for each species, — 
بو‎ thing which could not happen if they were derived from the fusion of 
sediments, since a heat which has not obliterated hornblende crystals 
was certainly not great enough to destroy all the materials of which our 
sedimentary rocks are composed. While the chemical composition of 
these rocks indicates a somewhat uniform origin, that of the sedimen- 
tary ones shows generally a mixed one, as we know is the case with the 
majority of them. 
The materials of the first division of the first class, too, in each species 
are only of such rock species as itself or of those that we know have 
preceded them in order of time; as, for instance, in rhyolite we find 
fragments of rhyolite, trachyte, andesite, and melaphyr or diabase, but, 
so far as I have seen, never an unaltered basalt. Rhyolite, being the 
most refractory of the volcanic rocks, is usually the most brecciated ; and 
