270 HENRY S. WASHINGTON 
The non-alteration in the very acid rocks is a fact which is 
difficult to explain by any of the hypotheses under considera- 
tion. Since the indications are that rhyolite when erupted is at 
a higher temperature than more basic rocks we could on the 
magmatic theories look for greater alteration in them than in the 
andesites or basalts. Since the reverse is the case their behavior 
militates rather against these theories. On the resorption theofy 
proper we could also expect to find in the acid magmas a greater 
tendency towards solution of the ferromagnesian crystals, as is 
hinted at by Rosenbusch,’ which we do not find. Glassy forms 
are extremely common in these rocks, and it is possible that the 
non-alteration is connected rather with this structural and phys- 
ical peculiarity than with their chemical composition. 
It is probably premature to discuss this question at present 
in view of the scantiness of our knowledge, so that we may 
merely assume that the large amount of SiO,, and perhaps K,O, 
exerts some deterrent effect on the process of alteration, of the 
nature of which we are ignorant. It is possible that the non- 
alteration in rhyolites is connected with the tendency of acid 
magmas to form hornblende and biotite rather than augite. It 
may also be suggested here that the influence of the composi- 
tion of the magma on the alteration process may be analogous 
to the influence of the nature of a solvent on the molecular con- 
dition of the dissolved substance.? 
CONSEQUENCES OF THE ALTERATION. 
Action of the magma.—Accepting the alteration and its attend- 
ant phenomena as facts we may examine some of the conse- 
quences of this change and see what light they throw upon a 
few petrological questions. 
The disrupting action of the moving magma current on the 
granular aggregate has already been referred to, yielding the 
rounded forms so commonly seen. This simple mechanical 
action of the current would tend to scatter the augite and 
* ROSENBUSCH, Mikr. Phys., Il., 660, 1887. 
2NERNST, Theoretical Chemistry. London and New York, 1895, 387. 
