258 HENRY S. WASHINGTON 
biotite, on the contrary, have only been formed artificially in the 
most recent years. As far as the experiments go they seem 
to show that the presence of a mineralizing agent (a fluoride or 
silicofluoride) is necessary for the production of biotite in the 
dry way, while hornblende has been produced only in the wet 
way very recently by Chrustschoff. The negative evidence of the 
experiments points to the conclusion, that for the formation of 
either biotite or hornblende in an igneous magma pressure is a 
highly important if not necessary factor, while of no moment in 
the formation of augite. 
CAUSE OF THE ALTERATION. 
Current theories—The theories in vogue to explain the altera- 
tion in question follow more or less closely Zirkel’s’ idea of a 
’ of the magma on the hornblende and 
biotite crystals, and suppose either a fusion of the crystals, or 
their “resorption” (solution) by the surrounding magma and 
the subsequent crystallization of the augite out of the magma. 
In regard to the magnetite there seems to be a difference of 
opinion, some supposing it to be not dissolved by the magma but 
crystallizing when the augitic constituents are withdrawn, while 
others seem to suppose it also to have been dissolved and recrys- 
tallized out of the magmatic solution. 
These theories postulate an instability of the hornblende and 
biotite, or at least their instability as regards the solvent power 
of the magma surrounding them, under surface conditions. 
While these various theories differ in details yet they may be 
discussed together, as the ideas involved in them are much the 
same and the objections to them are on the whole quite iden- 
tical. 
The current theories rest chiefly on the facts that the altera- 
tion proceeds from the outside inward and starts generally from 
crystal surfaces in contact with the magma; that the chemical 
composition of the magma has an apparent influence on the 
alteration, this being much less common in acid than in basic 
“caustic chemical action’ 
*ZIRKEL Mikroscop. Petrogr. U. S. G. Ex., goth parallel, VI, 95, 1876. 
