276 HENRY S. WASHINGTON 
in our knowledge of this group of rocks—a step which may lead 
to the extension of our knowledge in other directions. The ques- 
tion as to whether the knowledge of such an origin should affect 
the present, classification is a question apart and need not be dis- 
cussed here. 
Acta augite-andesites.— A consideration of the phenomena 
already discussed, as well as others to be mentioned presently, 
has led me to the conclusion that, besides the normal augite- 
andesites which make up the greater part, such derivative augite- 
andesites do exist, in which a large part of the augite present is 
due to the alteration of hornblende. What the grounds are for 
this view I shall endeavor to show, though my remarks must 
be taken as suggestive of what may be the case, rather than as 
even approaching a full discussion of the subject. 
| It is quite well established that hornblende and biotite are 
the prevalent, or one might say favorite, ferro-magnesian silicates 
of the more acid rocks, though egirine is abundant in acid 
phonolites and some trachytes, while augite is characteristic of the 
more basic rocks. This being so we could expect @ priorz to find 
the augite-andesites more basic than the hornblende or biotite- 
andesites. While this is the fact in many cases, yet exceptions 
are numerous, the abnormal acidity of many of the augite-ande- 
sites being remarked upon by Zirkel’ and Rosenbusch,? as well 
as by others. 
Some particular instances of the high acidity of augite-ande- 
sites occurring together with other andesites may be given. On 
the volcanic line in the A*gean Sea are found the two centers of 
Santorini, which has poured forth almost exclusively pyroxene- 
andesites, and A‘gina-Methana, where hornblendic andesites and 
dacites are abundant as well as pyroxene andesites. I have else- 
where3 pointed out the similarity in the general magmas of the 
two centers, and yet when we come to examine the rocks of the 
two districts in reference to the present point we meet with a 
t ZIRKEL, Lehrb. II, 802. 
2? ROSENBUSCH, Mik. Phys. II, 651-683, 1887. 
3JoUR. OF GEOL. III. 166, 1895. 
