ITALIAN PETROLOGICAL SKETCHES 25 I 



overburdened nomenclature, I shall designate these rocks as 

 biotite- V ulsi/i itc . 



The most important occurrence of biotite-vulsinite is that of 

 Mti. Sta. Croce and Lattani, which are, as far as can be seen, 

 entirely formed by a domal eruption of the rock just noted. This 

 was called " trachy-dolerite " by Abich,' he considering it a link 

 intermediate between trachyte and dolerite, while Pilla and von 

 Rath consider it a trachyte. Bucca calls it an augite-andesite, 

 and refers the greater part, if not all, of the feldspar to plagio- 

 clase. Otherwise his description agrees closely with mine. 



The Santa Croce rock is trach3?tic in appearance, having a 

 fine grained, but not very compact, light gray groundmass of 

 a slightly reddish tone. Through this are scattered very many 

 small, dark brown flakes of biotite, some small augites, and a few 

 small feldspars. The groundmass of the Mte. Lattani rock is 

 rather darker and of a bluish gray color, more compact, biotite 

 flakes are less abundant, and feldspars more common. My 

 specimen of this latter is not quite fresh. In thin section the 

 feldspar phenocr3^sts are quite common. They consist of an 

 alkali feldspar, apparently a soda-orthoclase, and a larger num- 

 ber of plagioclase, which is shown by its extinction angles to 

 have the composition AbgAng. These are well shaped, but con- 

 tain many inclusions of glass with small opaque grains and aug- 

 ite microlites. The biotites are of a greenish brown color, quite 

 fresh in the interior, but surrounded by a thin alteration border 

 of fine augite and magnetite grains. The large, irregularly 

 shaped augites are pale green. The groundmass is trachytic in 

 character and shows well-marked flow structure. There is an 

 abundance of small laths, chiefly of alkali feldspar, with some of 

 plagioclase. Magnetite grains and small diopside microlites are 

 present in abundance, and there is a residue base of alkali feld- 

 spar. 



The rock of Mte. Lattani closely resembles the above, though 

 there are certain differences, the rather more abundant feldspar- 

 phenocrysts are identical, as is also the augite. The biotites are, 



' Abich, Vulk. Ersch., Brunswich, 1841, 100. 



