THE VOLCANOES AND ROCKS OF PANTELLERIA 691 
ing original biotite crystals. The Ponza trachytes also differ 
chemically, especially in a higher potash content, judging from the 
only analyses of them which appear to have been made. As 
occurrences of these sodic trachytes are becoming rather common, 
it might be well to call the true Ponza type, with biotite, ponzite, 
and those of the Pantelleria type, gzbelite. 
PANTELLERITIC TRACHYTE, ZENETI TYPE (ZENETAL GRORUDOSE) 
Occurrence.—Rocks of this type, which are the “phonolites”’ 
of Foerstner, form flows seen in the lower portions of the internal 
scarps of the large, first-period caldera, notably at Costa Zeneti 
and Costa Zichidi. ‘The type also occurs near the base of the more 
precipitous portions of the coast line, as near Punta Pozzolana and 
Cala Cinque Denti on the north, the coast from Punta Tracino to 
Punta Kharace on the east, and various stretches on the south 
and southwest. These coastal occurrences presumably are exten- 
sions of flows, the more central portions of which form the bases 
of the internal scarps, and rocks of this type may be regarded as 
the earliest known outpourings of the Pantelleria volcano. 
Megascopic characters.—Rocks of this type are of a rather dark- 
gray color, and are usually compact, but slightly vesicular forms 
occur. The only phenocrysts visible are very small (1-2 mm.), 
glistening, white tables of feldspar, which are not conspicuous and 
make up less than 5 per cent of the mass. Here and there are 
equally small, black grains of augite, but the amount of these is 
quite negligible. The slightly brownish, dark-gray groundmass is 
dense and aphanitic, and in some specimens shows evidence of 
vitreous texture. 
Microscopic characters (Fig. 7)—A brief description of the 
microscopic characters of some of these rocks has been given by 
Rosenbusch,? which coincides with my own observations and the 
few statements of Foerstner.’ 
The most prominent phenocrysts are soda-orthoclase, though 
they are not large or abundant. They are euhedral and tabular 
.C. Doelter, Akad. Wiss. Wien, XXVI, 1875, pp. 148, 151. 
2H. Rosenbusch, Mrik. Phys., 11, No. 2 (1908), pp. 839, 967, I115. 
3H. Foerstner, Boll. Com. Geol. Ital., 1881, p. 534. 
