/ TA LI A N PE TR OL O GICA L SKE TCHES 833 



mens were hard to obtain. The other comes from near Vetralla 

 I 3""" southwest of Viterbo, from a quarry whose exact location I 

 could not ascertain. It may be from the same flow as the one first 

 mentioned since the two closely resemble each other. The sec- 

 ond is a light gray, harsh, typically trachytic rock. The ground- 

 mass is fine-grained and contains small biotite and augite pheno- 

 crvsts, with many glassy tabular feldspars. The sp. gr. of this 

 rock is 2.61 1 at ii°. The first is similar in character, but its 

 color is a light reddish brown due to atmospheric decomposition. 

 The feldspars are larger than in the other, and much resemble 

 the sanidines of the well-known Drachenfels trachyte. 



Under the microscope the two present much the same appear- 

 ance. The phenocrysts are of orthoclase with fewer of labra- 

 dorite, which contain few inclusions, the labradorite being 

 included in the orthoclase ; many well-formed phenocrysts of 

 pale green diopside, including patches of brown glass and some 

 magnetite; and some of brown biotite (especially abundant in 

 the second rock), which is much corroded and "altered," 

 though generally a core of unaltered substance remains. 



The groundmass is holocrystalline and typically trachytic. 

 Many small colorless or very pale green diopside prisms and 

 anhedra, with not abundant magnetite grains, lie in a paste of 

 alkali feldspar and fewer plagioclase laths showing flow struc- 

 ture. Between these laths in the second rock there is some 

 alkali feldspar as cement, which is almost entirely wanting in the 

 first. Stout apatite prisms with gray dusty inclusions are verj^ 

 common, and there are also a few small brown biotite flakes. 



An analysis of the Vetralla specimen is given in No. i 

 of Table II. Comparison will show the great resemblance 

 between it and that of the typical vulsinite from Bolsena.^ 

 Its mineralogical composition is also almost identical, though 

 labradorite takes the place of anorthite and is somewhat less 

 abundant apparently. It may be said that the rock was 

 thought to be rather a plagioclase-bearing trachyte till the 

 analysis showed its essential identitv with vulsinite. 



'This Journal, IV, 552, 1896. 



