PETROGRAPHY. 115 
HBRNICAL BRACCIANOSH. II. 7. 2. 2. 
Megascopic characters. Very dark gray or black, compact, aphyric, or only slightly por- 
phyritic. Augite phenocrysts rare, i to 3 mm., prismatic, black. Leucite phenocrysts very 
rare, i to 4 mm., equant, white or pale gray, inconspicuous. Groundmass very fine-grained or 
aphanitic. 
Microscopic characters. Holocrystalline, aphyric, few or no distinct phenocrysts, xeno- 
morphic granular fabric, leucite, augite, labradorite, orthoclase, nephelite, olivine, magnetite, 
apatite. 
Soda-orthoclase , Or 3 Ab 2 . About 5 per cent, anhedral, as formless interstitial patches. 
Labradorite, Ab t An 3 . About 15 per cent, anhedral, formless interstitial patches, multiply 
twinned. 
Leucite. 30 to 35 per cent, 0.05 to 0.30 mm., anhedral equant, less often irregular, inter- 
stitial between the augites, inclusions few, skeleton forms very rare. 
Nephelite. Groundmass: about 5 per cent, anhedral, formless, interstitial patches. 
Augite. 30 to 35 per cent, o. 10 to 0.50 mm., subhedral to anhedral, prismatic to equant, 
and irregular, greenish-gray, inclusions few. 
Olivine. About 2 per cent, 0.05 to o. 10 mm., anhedral, equant. 
Magnetite. About 3 per cent, 0.02 to 0.05 mm., anhedral, equant. 
Apatite. About i per cent, 0.05 to o.iomm., subhedral, prismatic. 
Chemical composition and norm as on p. 113. 
Type specimen from Arcioni, Rocca di Papa, Latian District. 
II. 7. 2. 2. Atrial Braccianose [Leucite-Tephrite, Atrio Type]. 
Megascopic characters. This type, which must be regarded as much less pre- 
cisely characterized than the preceding ones, occurs only as very small flows and as 
the borders and surfaces of larger ones, salbands of dikes, and crusts of bombs. The 
rocks are black or a very deep brown, evidently highly vitreous, and sprinkled with 
numerous small round spots of leucite, with only rare and small phenocrysts of feld- 
spar, augite, and occasionally olivine, the last only as an accessory. Vesicular and 
scoriaceous forms are common. 
Microscopic characters. In thin section these rocks show many small (up to i or 
2 mm.), highly euhedral and trapezohedral sections of leucite, sometimes with and 
sometimes without inclusions of glass, etc. Leucite skeleton forms are very common. 
Leucite makes up about one-quarter of the rock volume, but the relative amount may 
vary rather widely. Tables and prisms of labradorite (from Ab 2 An 3 to Ab I An 3 ) are 
present in less amount, not over 10 per cent, and also more or less euhedral pheno- 
crysts of greenish-gray augite, which are apt to be equant rather than prismoidal. 
The amount of these is small, about that of the feldspars. There may be also a few 
oli vines and magnetite grains, but always less than the two immediately preceding 
minerals. All these lie in a glass base, which is dark brown in color, either clear or 
dusty with indeterminable microlites, and which frequently shows flow textures. 
Chemical composition. An analysis was made of a specimen of this type, the 
material having been kindly furnished by Professor G. Mercalli, of Naples, who has 
published it elsewhere. 
