278 HENRY S. WASHINGTON 
Leaving these aside, we may confine our attention to the 
acid, ‘“‘trachytic,’ augite-andesites. In the magma of such a rock 
we could naturally expect the formation of hornblende or biotite 
rather than augite. 
Zirkel* explains the high acidity by the abundant presence of 
a glass basis which is often much more acid than the rock itself, as 
shown by Lagorio’s? analyses. This explanation, however, only 
shifts the difficulty one step back, or rather states it in another 
form, since Lagorio’s analyses also show that in basalts’ the 
basis contains almost exactly as much silica as the rock. 
The explanation which is suggested here is that these acid 
augite-andesites are derived from original hornblende or biotite- 
andesitic magmas. The magma came up with crystals of horn- 
'blende or biotite already formed, and these underwent alteration 
and disintegration as already described, while at the same time 
the rest of the ferro-magnesian molecules present in the magma 
crystallized as augite or hypersthene under the existing low pres- 
sure conditions. 
Against this view may be brought a number of objections, 
one of those which most naturally occurs being that in the very 
acid rocks, as the rhyolites, the tendency is for the hornblende 
and biotite to remain unaltered. This objection is of weight, 
but it must be remembered that the acid andesites in question 
seldom, if ever, attain to the high acidity of the rhyolites, and 
further, that they are chemically quite different otherwise. 
A detailed study of the various augitic andesites, and 
especially the more acid ones, would undoubtedly throw much 
light on the question and show which explanation (if either ) 
was correct. Such a study would, however, carry us beyond the 
bounds of this article, and is, moreover, impracticable for me at 
present. 
Variation with geological occurrence —I\t is evident, from the 
facts of the alteration, that the physical conditions under which 
* Z1RKEL, Lehrb. II, 819. 
2TaGorRIo, Min. Pet. Mitth., VIII, 458, 467, 1887. 
3 LAGORIO, op. cit., 479. 
