MAGMATIC ALTERATION OF CERTAIN MINERALS 267 
the highly complex chemical character of hornblende and biotite. 
This is in accordance with the well-known chemical law that com- 
pounds are more unstable the greater their molecular weight and 
the more complex their molecular structure. 
The theory here presented is that we must suppose the 
hornblende and biotite crystals to have been formed at an early 
(intratelluric) period under conditions of great pressure, and 
probably in the presence of mineralizing agents. As they are 
brought near the surface in the course of the eruption the pres- 
sure diminishes while the temperature remains high, till a point 
is reached where the substance is no longer stable. Herea 
molecular change is begun, inducing at the same time a molar 
change, so that the chemically and physically homogeneous 
mass of hornblende or biotite becomes a chemically and physi- 
cally heterogeneous granular mass of augite and magnetite. 
The formation of a mixture of diopside or augite and mag- 
netite out of hornblende or biotite substance is easily conceiv- 
able and chemically quite possible, in the case of biotite lime 
being taken from the magma. Similar changes are not at all 
uncommon in the range of chemical mineralogy, and the result- 
ing masses may be properly called paramorphs.'. Perhaps the 
best analogous example is that of the change of leucite to 
so-called pseudo-leucite, a mixture of orthoclase and nepheline, as 
described by J. F. Williams. In the case of hornblende, on 
this theory, the bulk analysis of the paramorph should be 
identical with that of the original mineral, or almost so (as is 
the case with pseudo-leucite). In the case of biotite which 
contains no lime (or, at least, only traces), that necessary for 
the formation of augite or diopside must be derived from the 
magma, while the potash of the biotite passes into this, or goes 
to form accessory feldspar. Such an interchange of atoms 
involves no theoretical difficulties, as similar ones are extremely 
common in cases of pseudomorphism and metamorphism. It 
may also be noted that the alteration of biotite produces larger 
*RoTH, Geologie, I. 64. 
2 Geol. Surv. Arkansas, 1890, II, 268. 
