Mineralogical Evolution. 245 
as explained by what the writer has named the crenitic 
hypothesis, have been elsewhere set forth at length, on - 
more than one occasion, and involve the whole chemical 
history of the various mineral species which enter into the 
constitution of rock-masses, but especially their relations to 
subterranean changes under the influence of heated water, 
and to atmospheric action. As we have pointed out, the 
transformation of basalt into the hydrous porodic body 
known as palagonite, and the subsequent partial conversion 
of this into a crystalline zeolite, as described by Bunsen, 
furnishes a significant illustration of the process under con- 
sideration. 
The stability of silicated species under atmospheric in- 
fluences is very variable, some being readily decomposed, 
and others very permanent; the indifference or chemical 
resistance, moreover, increasing with the hardness or 
mechanical resistance. These two qualities vary for species 
of analogous constitution directly as their condensation; 
while for species of similar condensation and hardness, the 
chemical indifference increases as alumina takes the place 
of the ordinary protoxyd-bases, lime, magnesia, ferrous 
oxyd and alkalies—a fact readily explained by the com- 
parative insolubility of alumina and aluminous silicates in 
atmospheric waters. The less partial action of dilute fluor- 
hydric acid on the various silicates shows more clearly 
than the atmospheric process, the relation of condensation 
to chemical indifference. This relation may be made 
evident by a few examples. The condensation being in- 
versely as the so-called atomic volume, we find that when 
calculated by a simple formula (elsewhere given by the 
author) for all silicates and oxyds, this value, represented 
by v (=p+d) for the various feldspars and scapolites, for 
nephelite, iolite, and petalite, equals 6°8-6.2; for the mus- 
covitic or non-magnesian micas, 5.9-5°6; for garnet, epi- 
dote, zoisite, and the various tourmalines, 54-53; for stau- 
rolite and spodumene, 4°9 ; and for andalusite, topaz, fibrolite, 
and cyanite, 5-0-4'5, approximately. Comparing with these 
the common protoxyd-silicates, we find for wollastonite and 
