MAGMATIC ALTERATION OF CERTAIN MINERALS 277 
somewhat striking result. In A‘gina-Methana the analyses of 
the hornblende-andesite (excluding segregations) show an aver- 
age silica percentage of 59.41, while at Santorini the pyroxene 
andesites are very acid, the silica average of the analyses quoted 
in the paper just cited, being 61.41 per cent., and the latest pro- 
ducts of eruption range from 66.15 to 68.89. 
Again we find in the Washoe District,’ the Yellowstone Park,? 
Colombia,3 and Ecuador‘ that eruption of the hornblende and 
pyroxene-andesites have taken place, and that many of the latter 
are as acid as, or more so than many, of the accompanying horn- 
blende-andesites. Other instances of the abnormal acidity of 
augite-andesites that might be quoted are not rare, and the fre- 
gency of such occurrences is certainly surprising in view of what 
we might @ priori expect. 
In 1881 Gimbel5 pointed out that the andesites of the Andes 
could be referred to two types, which he called the trachytic and 
the basaltic, the former with over, and the latter with under, 57 
per cent. of silica. Later, in 1883, Cross® suggested that the 
augite-andesites may be dividedinto three groups; the “trachy- 
tic’’ with a high percentage of silica, the “ basaltic’’ which are 
the most basic and pass into the true basalts, and the ‘“inter- 
mediate”’ 
to 60. 
That the “intermediate” and “basaltic” augite-andesites have 
had the greater part at least of their augite crystallized out of 
the magma in a normal way, and hence that they are really 
normal augite-andesites with a silica percentage of 56 
augite-andesites in a true sense of the word, seems to be generally 
the fact; though cases might be brought up in which a deriva- 
tive origin could be rightly attributed to a part of the augite 
even of these. 
* HAGUE and IppInGs, Bull U.S. G. S. No. 17, p. 33. cf also Cross, Laccolite: 
Mt. Groups, XIV, Rep. U.S.G. S. 227, 1895. 
2 IDDINGS, Electr. Peak, XII Rep. U.S. G. S., 627, 628, 1892. 
3 Kiich, Petr. vulk. Gest. Rep. Colombia, Berlin, 1892, 78, 79. 
4Siemiradzki, op. cit., 205, 206. 
5 GUMBEL, Sitz. ber., Miinch. Akad., XI, 365 ff., 1881. 
SGross; bull NO. iy Wi. iSa Ge 5:5)305, 375 1683s 
