SKETCH OF AEGINA AND METHANA. I41I 
The not very abundant plagioclase phenocrysts seem to be 
identical with those of the surrounding rock, showing twinning 
lamellz and carrying the same inclusions, though usually those 
of glass do not form a distinct ‘‘net.”” One group of large plagio- 
clases includes numerous patches of brown biotite all of which 
are oriented alike (micropoikilitic structure), and a little biotite 
also acts as a mesostasis between the plagioclase crystals. The 
larger augite crystals, which are to be classed as phenocrysts, are 
quite abundant and do not differ materially from those of the 
surrounding rock. Some biotite crystals, brown in color and 
wholly altered, are also seen. 
The hornblende which is present in abundance as phenocrysts, 
though it also bears much resemblance to that of the mother 
rock, calls for special description. The crystals are all prismatic 
and, where the original hornblende substance is seen, of the usual 
brown color, though rather darker in tone. In many of the 
fresh patches of unaltered hornblende are found narrow bands of 
much darker color. These bands, which even with the use of the 
highest powers are irresolvable, show the pleochroism of the reg- 
ular brown hornblende, though modified by the greater depth of 
color. In hornblende sections cut normal to the vertical axis. 
they lie parallel to the clinopinacoid, bisecting the obtuse angles 
of the cleavage lines, while in longitudinal sections (approxi- 
mately parallel to 6(o10)) they cut the parallel cleavage lines 
obliquely in the direction of a positive orthodome. The angle, 
which is not quite constant, is in general 65°—70°, making the 
bands lie parallel to the plane f (301). These bands are hence 
seen to represent sections, not of sheets, but of rods, similar to 
but thicker than those seen in the hornblende of a rock from Mt. 
Pagoni. 
The alteration of the hornblende, however, is its most constant 
and characteristic feature. The crystals have all undergone a 
change, so much so that kernels of fresh substance are compara- 
tively rare. In most cases they show a mass, or else a central 
core, of fine grains of black opacite embedded in a finely granular 
mass of augite and plagioclase. These occurrences show clearly 
