THE 
JOURNAL G2 Crome 
APRIL-MAY, 1806. 
THE MAGMATIC ALTERATION OF HORNBLENDE 
ANINID) JEON, 
Iv is proposed in the following pages, which may be regarded 
as an elaboration of certain ideas expressed by me a few years 
ago, to discuss the causes of the well-known ‘“magmatic altera- 
tion” or “resorption” of hornblende and biotite, and to suggest 
the possibility of certain consequences which seem to follow on 
this alterationt and which have an influence on the mineralogical 
composition of certain rocks. 
The alteration of the two minerals in question to a granular 
mass of augite and magnetite is so well known to every petrog- 
rapher, and such an excellent detailed description of it may be 
found in Zirkel’s Lehrbuch (u, 716 ff.), that it is unnecessary to 
devote any space to a description of the facts. It must be 
recalled, however, that analogous alterations are extremely rare 
and of doubtful character in the pyroxenic minerals. 
The important facts are also to be remembered that the 
chemical composition of both hornblende and biotite are struc- 
turally much more complex than that of the pyroxenes, and that 
they contain a greater variety of atoms, hydrogen and fluorine 
being very frequent, if not constant, constituents of both. 
It must further be noted that augite is of all minerals one of 
the easiest to produce artificially either by dry igneous fusion or 
in other ways, and that melted hornblende invariably crystal- 
lizes, under laboratory conditions, as augite. Hornblende and 
‘The word aéferation as used throughout this paper will be understood to mean 
only this ‘** magmatic” alteration or “resorption.” 
Vor LV:NG. 3: 257 
