262 HENRY S. WASHINGTON 
altered crystals predominate largely over the unaltered ones, 
occasionally to the total or almost total disappearance of the 
latter: 
When we come to the acid and basic rocks we find that, while 
in both the alteration is comparatively rare, yet that there is a 
radical difference in the cause of the rarity in the two cases. In 
the basic magmas there are large amounts of iron oxides, 
magnesia and lime which here crystallize as pyroxene and olivine 
rather than as hornblende and biotite, which only rarely occur. 
When these two do occur, however, as in the hornblende basalts, 
they are almost invariably altered, so that the rarity of the altera- 
tion in these rocks is due to the rarity of the alterable minerals. 
The case is quite otherwise in the acid rocks, where the 
-bivalent metals, always in small quantities, go to form hornblende 
and biotite, pyroxene being of rare and abnormal occurrence in 
the rhyolites. Yet, notwithstanding their comparative frequency, 
though in small amounts, the alteration of these minerals is very 
unusual, the biotite being only occasionally blackened on the 
edges, while the hornblende is seldom changed. 
We find in fact that while the absolute number of altered 
crystals is greatest in the andesites, yet that the proportion of 
altered crystals of hornblende or biotite among all those present 
is greatest in the basalts, where it reaches nearly 100 per cent., 
and decreases gradually, through the various intermediate rocks 
to the rhyolites where it is almost zero. 
The general facts show that the chemical composition of the 
magma is a factor in the alteration process, though the role it 
plays is a rather obscure one. That it is a causal factor, 7. ¢., 
that the relative basicity of the magma in the intermediate and 
basic rocks immediately induces the alteration, does not seem to 
be probable in the light of facts to be given further on. It seems 
to be rather modifying in its action, the alteration which would 
Bd., IV, 213, 1886; Buaas, Persia, Min. Pet. Mitth., III, 474, 1880; Hatcu, Arequipa, 
ditto, VII, 208-360, 1886; RUDOLPH, Peru and Bolivia, ditto, IX, 294, 304, 1887; K. 
VOGELSANG, Eifel, Zeit. d. d. g. G., XLII, 13, 18, 1890; GrossER, Siebengebirge, 
Min. Pet. Mitth,, XIII, 77, 1892; G. H. WILLIAMS, Fernando Noronha, Am. J. Sci., 
XXXVII, 184, 1889. 
