MAGMATIC ALTERATION OF CERTAIN MINERALS 271 
magnetite grains through the groundmass, and the process may 
often be seen, so to speak, in operation, where a tail or streamer 
of such grains is observed proceeding from a rounded, altered 
crystal. 
It is not to be supposed that these grains will be resorbed or 
dissolved by the magma under ordinary conditions. The pres- 
sence of augite microlites and crystals which have been formed 
down to the last stage before solidification, and the accretions 
about augite phenocrysts, precludes any such idea for the augite. 
Iddings * has shown that where resorptive action takes place the 
last minerals to crystallize should be the first to be resorbed, so 
that the magnetite grains are also safe from the attack of the 
magmatic ogre. It follows, therefore, that, provided the mass 
be in sufficient motion, the greater the alteration of hornblende 
and biotite the greater will be the abundance of augite and 
magnetite grains in the groundmass. 
This conclusion may be supported by numerous observations : 
H. Vogelsang? was the first to suggest that the ‘“‘opacite”’ 
grains in the groundmass were derived from altered hornblende 
and biotite crystals by mechanical disintegration. 
Zirkel3 concludes that ‘‘many of the dark grains scattered 
through rocks are really the finely-distributed, powder-like par- 
ticles of the pyrogenous alteration product of hornblende.”’ 
Mugge,* in his paper on the rocks of the Azores, makes the 
definite statement that augite grains as well may have been 
derived from altered hornblende and biotite crystals. We have 
also the interesting fact shown that the augites which crystallize 
out of the magma may be, and are in this case, of a different 
composition. On page 224 he says that certain groundmass 
olivines ‘‘appear in these cases, as sometimes in trachytes else- 
where, to have been produced from hornblende and biotite by 
resorption (Umschmelzung).” 
tI DDINGS, Cryst. Ign. Rocks, Bull. Phil. Soc., Washington, XI., 105, 1892. 
?H. VOGELSANG, Philosophie d. Geologie, Bonn, 1867, 192, cf. also his Die Kry- 
stalliten, Bonn, 1875, 157. 
3 ZIRKEL, Mikr. Pet. goth Parallel, Washington, 1876, 95; also Lehrb., I., 717. 
4MuGGE, Neu. Jahrb., 1883, II., 1895. 
