PETROGRAPHY. 103 
Augite. Microphenocrysts and microgroundmass : about 20 per cent, o. i to 0.5 mm., 
subhedral to anhedral, stout prismatic and equant, pale greenish-gray, non-pleochroic. 
Olivine. Microgroundmass: about 5 per cent, 0.08 to o. 10 mm., subhedral to anhedral, 
equant, colorless. 
Barkevikite. Microgroundmass; about 2 per cent, 0.02 to 0.05 mm., anhedral, as inter- 
stitial areas or fringing the augites, light brown. 
Magnetite. Microgroundmass: about 2 per cent, 0.02 to 0.05 mm., anhedral, equant. 
Apatite. Microgroundmass: about i per cent, subhedral, prismatic. 
Chemical composition and norm as on p. 101. 
Type specimen from Monte Cavallo, south of Orvieto, Vulsinian District. 
II. 7. 2. 2. Vesbal Braccianose [Leucite-Tephrite, Vesuvius Type]. 
Megascopic characters. Rocks of this type are of a general dark-gray color, 
speckled with numerous small white spots. They are highly porphyritic, the very 
great majority of the phenocrysts being of leucite, and with small and almost negli- 
gible numbers of phenocrysts of augite and sometimes of olivine or biotite. The 
leucite phenocrysts vary in diameter from i to 3 mm., being seldom over the latter. 
They are for the most part spheroids, giving round sections, but are also well-formed 
trapezohedra. In color they are usually pale gray, rather than clear white, so that 
they are not as strikingly conspicuous as might be expected, though always clearly 
discernible. The few augite phenocrysts are dark green, in small, stout prismoids 
and grains, seldom over 3 mm. long. The more uncommon olivine phenocrysts 
are of similar-sized, yellowish-green grains, and the still rarer biotites form small 
black tables. The groundmass is a dark gray, compact and aphanitic, and in the 
field these rocks would be called leucite melaphyres. Most of the specimens are 
more or less vesicular. 
Microscopic characters. In thin section the leucite phenocrysts, which form 
nearly one-half of the rock volume, present mostly rounded outlines, with occasional 
crystal planes. They are apt to be cracked, or sometimes in clusters of several closely 
juxtaposed individuals. Double refraction is not well marked, as a rule, and inclu- 
sions are generally few and irregularly arranged. An occasional stout and pris- 
matic labradorite phenocryst is seen, though these are seldom if ever visible in the 
hand specimen. The augite phenocrysts are in stout prismoids, usually much 
broken and often anhedral, of a slightly yellowish-brown tinge of gray. They 
carry few inclusions, and these mostly of magnetite. The rare, anhedral, generally 
fragmentary olivine phenocrysts call for no comment, nor do the still rarer tables of 
brownish biotite, which show the common alteration phenomena. 
The groundmass in which these lie is usually holocrystalline, but a trifling 
amount of glass is visible in some instances. The fabric is xenomorphic granular, 
somewhat intersertal through the development and diverse arrangement of the labra- 
dorite, and may be briefly described as subintersertal. It consists in large part of 
pale-gray, slightly brownish augite, sometimes showing a zonal structure, in irreg- 
ular anhedral grains, few of which assume a distinctly prismatic habit. Tables of 
labradorite, which give rise to lath-shaped sections, are fairly numerous, their length 
