SKETCH OF A.GINA AND METHANA. 145 
giving long rectangular sections, and are mostly perfectly water- 
clear. They do not show many twinning lamelle and occasion- 
ally have undulatory extinction, but are all certainly plagioclase. 
The extinction angle of 35° observed in one case indicates that 
they are anorthite, which is confirmed by the high percentage of 
CaO found on analysis (cf. Anals. 8, 11, Table 1 ae ee tewalance 
stout plagioclase crystals are seen, the megascopic phenocrysts, 
which resemble those of the surrounding rock. They all show 
a core of glass network, or of fine plagioclase grains with very 
little glass, resembling the holocrystalline xenomorphic ground- 
mass of some porphyries. These crystals often enclose flakes of 
hornblende. 
In the reddish hypersthene-andesite of the Methone acrop- 
olis are found round, dark gray fine-grained segregations, which 
on examination under the microscope show no marked difference 
from the rock surrounding them. They also are hypersthene- 
andesites, with some accessory brown hornblende phenocrysts 
and many small hypersthene prisms in the vitreous groundmass, 
which shows a beautiful flow structure. 
It is noteworthy that in all the segregations magnetite, 
except that due to alteration of the hornblende, exists only 
sparingly, it being entirely absent in one or two specimens. It 
is rather more common in those with green hornblende in rather 
large grains, which are more frequently found as inclusions in the 
hornblende than in the plagioclase. Zircon, on the contrary, is 
rather common in all the segregations as small colorless crystals, 
both included in the other minerals and in the groundmass. 
These masses show no evident signs of the action of the sur- 
rounding molten magma upon them (except in their rounded 
outlines), and in the enclosing rock itself no traces are to be 
seen in their immediate vicinity of any change in structure or 
composition. The presence of glass base in them (also observed 
by Kuch and Belowsky) is interesting and, inasmuch as it is 
impossible to conceive of the masses having preserved their 
*For such contact phenomena in the case of enclosures, cf. DANNENBERG, Min. Pet. 
Mitth. XIV., 1894, pp. 17-94. 
