THE VOLCANOES AND ROCKS OF PANTELLERIA 701 
Microscopic characters (Fig. 8).—The pantellerites of Pantelleria 
have been described in great detail by Rosenbusch,’ who devotes 
especial attention to this type. Except for some minor details my 
observations fully bear out his descriptions. 
-The soda-microcline phenocrysts are not numerous in the 
sections. In general they are thick, tabular parallel to b (oro), 
and euhedral, but are often fragmentary. They show no micro- 
cline or perthite structures and no albite twinning lamellae, but 
Carlsbad twins are frequent. Inclusions are rare. 
In most of the specimens cossyrite phenocrysts are fairly 
common, while in others they are wholly wanting. They are 
stoutly prismatic and vary from 0.2 too.4 mm. in length. Origi- 
nally euhedral, most of them show rounded outlines, especially at 
the terminations, and some are thus reduced to ovoidal forms. 
This is probably due to resolution by the magma, as suggested 
by Rosenbusch. Their color is a very deep reddish brown, with 
internal pleochroism; r¢ brownish black, deep chestnut brown, 
a red brown. A few small phenocrysts of a pleochroic, grass- 
green aegirite-augite are present. Magnetite grains are wholly 
absent, but there are a few zircons. I could find none of the olivine 
or apatite mentioned by Rosenbusch. 
This rock type has a very peculiar and highly characteristic 
groundmass. Under low powers it is light, yellowish white and 
almost opaque through the abundance of patches and streaks of a 
dustlike substance, which commonly show a well-developed flow 
texture. This invariably occupies most of the section, and in the 
narrow interstices is a colorless substance with feeble, diffuse 
birefringence. 
Under high powers the opaque substance is resolved into a 
felt of extremely minute, transparent, nearly colorless, prismatic 
microlites. These must be considered to be aegirite (as suggested 
by Rosenbusch), though their extreme tenuity renders determina- 
tion of their optical characters very difficult. It is, of course, to 
the presence of this aegirite felt that the greenish color of the rock 
is due. No cossyrite or other hornblende was observed as a ground- 
mass constituent. The colorless base is now seen to be generally 
holocrystalline, and formed of an intimate mixture of very minute 
tH. Rosenbusch, op. cit., pp. 852-54. 
