266 HENRY S. WASHINGTON 
does not carry with it the implication that augite as well may 
not be formed under the same conditions (as Siemiradzki seems 
to suppose ). On the contrary, many facts, such as the inclusion 
of augite in phenocrystic hornblende and the like, show conclu- 
sively that augite can be formed under the same conditions as 
hornblende. 
It would be perhaps rash to say that under plutonic condi- 
tions the bivalent molecules tend to form hornblende and biotite 
rather than pyroxene, but a hint that this is the case is furnished 
by the general preponderance of these two minerals over pyrox- 
ene in the plutonic rocks. The predominance is not, certainly, 
as great as might be wished to establish this point clearly, but 
that it exists is quite evident on considering the much greater 
abundance of the hornblende and biotite bearing plutonic rocks 
over the augitic; the granites, hornblende and biotite syenites 
and diorites surpassing, both in quantity and in number of 
occurrences, their augitic varieties as well as the gabbros in the 
broader sense. This question, however, which has no especial 
bearing on the present hypothesis, is merely brought in paren- 
thetically." 
It is evident from their freedom from alteration in the plu- 
tonic rocks that hornblende and biotite are stable under plutonic 
conditions down to the last moment of solidification, while their 
constant alteration in the majority of volcanic rocks shows that 
they are unstable under diminished pressure, a fact that the 
experiments of Becker’ and Doelter and Hussak3 tend to con- 
firm. 
This instability * under conditions of high temperature and 
diminished pressure is due, according to my view, directly to 
‘It may be mentioned that Brogger (Gror. Ting. Serie, Krist. 1895, 36) states 
that in the grorudites the hornblende evidently represents an older phase of crystal- 
lization than the eegirine. 
? BECKER, Neu. Jahr., 1883, II, 1 ff. 
3DOELTER and HUSSAK, ditto, I, 1884, 23. 
+ ROSENBUSCH has already recognized this idea of instability, but without assign- 
ing a reason for it (Mikr. Phys., II, 660, 1887). Cf G. H. WiLuiaMs, Am. J. Sci., 
XXVIII, 259, 260, 1884. 
