268 HENRY S. WASHINGTON 
magnetite grains and less augite than that of hornblende,’ which 
is quite in accordance with its greater content of iron and lack 
of lime. No chemical difficulties are encountered in the case of 
the accessory alteration products, hypersthene, olivine and feld- 
spar, the molecules necessary for their formation being present 
in the mother mineral. 
The process requires apparently considerable time, and we 
may suppose that it continues from the point where instability 
begins till the mass becomes solid. It is possible that it may 
continue for some time after solidification has set in, but of this 
we have no evidence. Whether the magma is in motion or at 
rest has no effect on the alteration process, though the effects 
produced by a moving magma on the alteration product may 
have important consequences, as we shall see. 
The formation of augite may take place at the same time as 
the hornblende, but is not hindered by diminution of pressure, 
and may possibly be favored by it. So that augite may and 
does crystallize out of the magma, either as small groundmass 
crystals or microlites, or as zonal accretions around phenocrysts 
brought up from below, at the same time that the alteration of 
hornblende and biotite is going on, or even after this has ceased 
for lack of alterable material. 
The relative chemical complexity explains satisfactorily the 
fact that analogous alterations are seldom, if ever, seen in either 
the monoclinic or orthorhombic pyroxenes, or in olivine, which 
are much less liable to change, owing to their comparatively 
simple composition and structure. This also explains the 
recrystallization of melted hornblende as augite. 
It will be evident that this theory of simple molecular 
change involves no change of form such as would necessarily 
arise on fusion or solution of the crystal. The preservation of 
the original shape is in accordance with the facts of pseudomor- 
phism, and it is easily conceivable that the fine grained product 
should possess in many cases sufficient cohesion between its 
closely packed and interlocked grains to resist considerable dis- 
1 ZIRKEL, Lehrb., I, 721. 
