SKETCH OF 4,GINA AND METHANA. oy 
in the thinnest spots, a granular structure, the grains being semi- 
_transparent, of a chestnut-brown color and isotropic between 
crossed nicols. > 
The fresh appearance of the rock and the crystals, and the 
absence of a surrounding border of brown limonite, seem to pre- 
clude the idea that their appearance is due to weathering, though 
they somewhat resemble the limonite pseudomorphs after mag- 
netite observed by von Lasaulx* in some altered basalts from the 
Auvergne. In color and physical characters the grains bear a 
great resemblance to the crystallites in the Bobenhausen 
tachylyte* which Zirkel? holds to be an extremely ferruginous 
glass. It may be, then, that these large magnetite crystals are 
in reality pseudomorphs of such a glass after the original mag- 
netite, due to the combination of the latter with the magma. 
A curious fact is that the majority of these larger magnetite 
crystals are accompanied by perlitic cracks in the groundmass, 
which surround the magnetite at a short distance. from it either 
completely or partially, there being sometimes only one and 
sometimes several such cracks about each grain. These cracks 
are not met with elsewhere in the groundmass, nor do they sur- 
round any of the other minerals, and their formation may be 
connected with the brown coloration and granular structure just 
described. It must be noted that the groundmass between the 
cracks and the magnetite grains, quite up to the latter, is entirely 
normal and identical in ‘color, structure, etc., with that elsewhere. 
In addition to their green hornblende these andesites are 
specially marked by the presence of tridymite, which is very 
common, especially in the rocks of the Stavro district. This 
oceurs in’small irregularly-shaped masses or round rosette-like 
clusters showing the usual shingle structure, or in radially 
arranged spherical aggregates, about 0.1 mm. in diameter. As 
they often occupy the sides of small cavities they seem in part 
to be of secondary origin, but this is not at all certain. 
*Neu. Jahrb., 1870, p. 695. 
2 Cf. VOGELSANG, Die Krystalliten, Bonn, 1875, p. ili., and Pl. XIV., Fig. 1. 
3 ZIRKEL: Basalt Gesteine, Bonn, 1870, p. 182, ff. 
