SKETCH OF A.GINA AND METHANA. Oy, 
tinge, with feeble but marked pleocroism, || c pale greenish 
gray, |c pale yellow. The hypersthene is the most easily 
decomposable constituent of these rocks, perhaps owing to the 
large amount of FeO,and in the more altered rocks (such as 
those of Mt. Oros) they have almost entirely disappeared, and 
are only represented by red and brown rectangles and patches 
of ferrite, or other ferruginous decomposition products." 
The plagioclase lathes and crystals of the groundmass show 
few planes and are usually rectangular in outline, They are 
either single crystals or simple twins, very few showing multiple 
twinning lamella. As they possess extinction angles of 28° to 
31° they belong to the bytownites richer in lime, or even to the 
anorthites. 
A striking characteristic of the hypersthene-andesites proper 
is their extreme paucity in phenocrysts, especially of the ferro- 
magnesian silicates. Plagioclase phenocrysts occur, but not 
abundantly, both megascopically and microscopically, but 
phenocrysts of pyroxene, hornblende or biotite are almost 
entirely lacking; a circumstance which, together with the facts 
that hypersthene is abundant in the groundmass, and that the 
small plagioclase lathes are apparently richer in lime than the 
phenocrysts, forms another occurrence, in addition to the many 
already noted, which are not in accordance with Rosenbusch’s 
law that as crystallization proceeds the remaining magma becomes 
more acid. 
Phenocrysts are, however, much more abundant in the other 
sub-group, and consist chiefly of augite in not well shaped, clear, 
and almost colorless crystals. _ Hypersthene in prisms up to 0.8 
mm. in length also occurs. The presence of augite seems to have 
determined, or to be correlative with the presence of hornblende 
which is almost or quite entirely wanting in the hypersthene- 
andesites, but rather abundant as phenocrysts in the other group. 
Originally of a brown color it is invariably altered to an augite 
opacite aggregate, which is generally quite fine-grained and sur- 
rounded by an outer zone of colorless augite with some plagioclase. 
‘Cf. FELIX and LENK: Beitr. Geol. Pal. Rep. Mexico, Leipzig, 1890, I., p. 91. 
