THE VOLCANOES AND ROCKS OF PANTELLERIA 685 
busch,’ with whose observations my own coincide in nearly all 
particulars. 
The most abundant and largest phenocrysts are of soda- 
microcline, in thick tables, parallel to 6 (o10), or in stout prisms 
elongated parallel to the vertical axis, either euhedral or subhedral, 
and often fragmentary. Some of these show a microperthitic 
structure in very fine lamellae, which resembles the lamellar 
twinning of the soda-lime feldspars and accounts for Foerstner’s 
designation of “‘andesite.’”’ Typical microcline grating-structure 
is rare in my specimens, but Carlsbad twinning occurs. In many 
cases the feldspar phenocrysts are free from inclusions, but in 
others, especially from Monte Gibelé, there are inclusions of augite 
(which is often poikilitic and extinguishes simultaneously in 
isolated patches). 
A few small phenocrysts of augite are present. They are 
mostly stout subhedral prisms, either colorless or light greenish 
and non-pleochroic. Aegirite-augite is either wanting or very 
rare, but occurs in the rocks from Costa Zichidi and its neighbor- 
hood. No phenocrysts of either hornblende or biotite are present, 
and small phenocrysts of olivine were only rarely seen, though 
Rosenbusch notes this mineral as an almost constant phenocrystic 
accessory. Small irregular grains of magnetite are fairly constant, 
but always in small amount, this type and the basalts being the 
only rocks of the island in which it occurs. 
The typical groundmass is holocrystalline, but a little glass 
may be present occasionally, and some of the Gibelé lavas are 
quite vitrophyric. Soda-microcline makes up the greater part, 
either in short, small laths or anhedra, both being present in the 
same specimen. A flow texture is more or less well developed, 
though this seldom assumes a typically trachytic fabric. 
The mafic? minerals in the groundmass are pyroxene and horn- 
blende. The former is a colorless augite, less often greenish and 
aegiritic, in prisms or more abundant anhedra. No hypersthene 
* H. Rosenbusch, Mikr. Phys., II, No. 2 (1908), p. 1115; on p. 926 he speaks of 
them as trachytes. F 
2 Equivalent to ferromagnesian; cf. Cross, Iddings, Pirsson, and Washington, 
Jour. Geol., XX (1912), 560. 
