32 IEE J OOWKINAUIL QUE Cle OULOG Ye. 
hornblendes of the hornblende-andesites or than the augite crys- 
tals accompanying them, and to have been originally well shaped 
stout crystals, without the orthopinacoid. The color is a pale 
yellow-brown, with strong pleochroism; ¢ yellowish brown, 6 
yellowish brown, @ colorless, the absorption being ¢ >) > a. 
In some of the hornblende-augite-andesites of Mt. Pagoni an 
intermediate type is seen, of a greenish brown color, which, how- 
ever, resembles more the brown than the green variety. The 
extinction angle of the brown variety is lower than that of the 
green, varying from 5°—6° 35’—~a fact quite in accordance with 
the observations of Belowsky, to be mentioned later. Inclusions 
are not common. 
This brown hornblende, as has been so often noticed, is decid- 
edly prone to alteration. While most of the small crystals which 
occur in the groundmass are either quite fresh or only altered on 
the edges, the larger crystals, even the largest, which may be ten 
or many more times the diameter of these smallest ones, are 
entirely altered (or with only a small unaltered core) to a very 
fine-grained almost opaque, black mass of augite and opacite 
(magnetite) grains. The further stage of alteration to a more 
coarsely granular augite-magnetite aggregate is not generally 
seen in these rocks. Notwithstanding their alteration the orig- 
inal sharp outlines of the hornblende have been excellently 
preserved. 
This alteration is so familiar and has been so often described 
that it is unnecessary to dwell upon it here at any length. One 
or two points call however for special mention. The presence 
of the small, unaltered crystals along with much larger altered 
ones points to the conclusion that not only were the large ones 
altered at a quite early period, but that the small ones were 
formed subsequently to, and under different conditions from, the 
alteration of the larger ones. 
I observed also in many cases of juxtaposition between horn- 
blende and feldspar that the presence of the feldspar had no effect 
on the alteration, this being as great at the surface of contact as 
along the free sides of the hornblende crystal. This differs some- 
