SKETCH OF AEGINA AND METHANA. 33 
what from the observation of Kuch? that hornblende in contact 
with microfelsitic groundmass was unaltered, while next to vitreous 
groundmass it had been altered, this being seen even in the same 
crystal. = 
A few words in regard to the difference between the brown 
and the green varieties of hornblende. Belowsky,? on the basis 
of some very valuable and interesting experiments of his own, 
puts forward the view that the brown is chiefly produced by 
alteration of the green ‘‘through the action of heat, in which 
oxidation was possible.” Against this view must be brought up 
the fact that here in A‘gina we meet with both varieties, but 
never in the same rock species. The green is characteristic of 
the pyroxene-free hornblende-andesites, and where this has under- 
gone alteration, as in the case from Mt. Chondos, described on page 
25, the resulting form of hornblende is quite different from the 
brown variety just described, which is the characteristic horn- 
blende of the pyroxene-bearing andesites. This correlation of 
almost colorless pyroxene and brown hornblende, which is well 
established in the rocks before us (and, I may add, in the closely 
analogous augite-andesites of Smyrna and biotite-andesites of 
Pergamon), shows that the formation of the different varieties 
of hornblende is dependent on the different chemical constitu- 
tions of the magmas, and on the different conditions, which in 
one case determined the crystallization of the ferro-magnesian 
molecules as pyroxene and brown hornblende, while in the other 
they crystallized out as green hornblende, with or without biotite, 
and with little or no augite. 
That the green hornblende on heating assumes the color and 
extinction angle of the basaltic hornblende is not to be denied, 
but in view of the facts above mentioned, and the vague and 
unproven character of the difference of heat conditions in the 
formation of the different varieties of hornblende, it is surely too 
much to infer from this fact that the brown hornblende is always 
derived from the green by alteration of the latter, and cannot 
have an independent origin and existence of its own. 
DCH Op ycit-epa S5-au ela Wen ig O: 
?BELOWSKY. Op. cit. p. 37 ff. 
