282 HENRY S. WASHINGTON 
magma and a statement of certain objections to them, the con- 
ditions under which the alteration does or does not take place 
are described and the conclusion arrived at ‘‘that a diminution of 
pressure together with a high temperature continued for some 
time are the conditions necessary for the alteration.” 
The theory is then proposed that the two minerals are stable 
in a molten magma only under plutonic conditions, and that the 
alteration is simply a molecular rearrangement induced by the 
diminished pressure in volcanic conditions. 
The instability of hornblende and biotite as compared with 
pyroxene is due to their much more complex chemical constitu- 
tion. 
The influence of the chemical composition of the magma is 
_ described, but a discussion of it is held to be premature at 
present. 
The disrupting action of the surrounding moving magma in 
the granular alteration product is touched upon, and it is pointed 
out that many of the groundmass augites and magnetites must 
have been derived from altered hornblende or biotite crystals. 
It is also suggested that some of the augite-andesites, especi- 
ally the more acid ones, may owe their augitic character to the 
alteration and subsequent disintegration of previously existing 
hornblendes or biotites, in accordance with the views of Von 
Lasaulx. 
Finally, several applications of this idea to the explanation 
of certain rock occurrences and the succession of volcanic rocks 
are given. HENRY S. WASHINGTON. 
