70 THE ROMAN COMAGMATIC REGION. 
of the fiescolal type study of the norm and mode shows clearly that the presence of 
leucite in the rock to a notable extent would be most improbable on account of 
the presence of low alkalis, especially in view of considerable amounts of ferrous 
oxide and magnesia, which would allow the establishment of a silica equilibrium by 
the formation of olivine and hypersthene, the latter entering the augite molecule. 
Occurrence. This type is not very abundant, but probably more so than the 
number of specimens which I collected would indicate. It seems to be confined to 
the Vulsinian and Ciminian districts, prominent localities being the quarries at 
Bagnorea in the former and the flows of Monte Venere and one on the south shore 
of Lake Vico in the latter. 
Name. The name of the type is derived from that of the locality of Bagnorea, 
near Orvieto, where the rock is quarried extensively for paving stones. 
In the prevailing systems the names commonly bestowed on these rocks vary 
with different observers, according to the particular minerals on which stress is laid. 
Thus the Bagnorea rock was formerly called by me leucite-phonolite on account of 
the nephelite cement present, while Bucca calls it a leucite-trachyte and Modern! 
a leucitophyre. Similarly the rock of Monte Venere was called by me a leucite- 
trachyte, while Mercalli and Sabatini consider it a leucite-tephrite, on account of 
the presence of labradorite, even in small amount. As may be gathered from the 
description above, and as is more evident from the discussion of the mode, the 
amounts of both labradorite and nephelite are so small that it would seem unwise, 
in bestowing a name, to allow their presence to offset the much more abundant 
orthoclase. On this account, in the prevailing systems, the name of leucite-trachyte 
is the most appropriate. 
BAGNOREAL CI.MINOSE. II. 5. 2. 2. 
Megascopic characters. Light gray, compact, porphyritic. Leucite phenocrysts few, 
i to 5 mm., irregular, inconspicuous. Augite phenocrysts few, i to 2 mm., black, prismatic, 
inconspicuous. Biotite phenocrysts few, i to 2 mm., tabular, bronzy, sometimes wanting. 
Groundmass light gray, aphanitic. 
Microscopic characters. Holocrystalline to percrystalline, megaporphyritic, dopatic. 
Phenocrysts: 10 to 15 per cent, leucite, augite, biotite, orthoclase, labradorite. Groundmass: 
90 to 85 per cent, orthoclase, labradorite, leucite, augite, magnetite; also olivine, biotite, and 
nephelite, not always present. Glass base rare. 
Orthoclase, from Or 2 Ab t to OrjAbj. Phenocrysts: about 2 per cent, not notable mega- 
scopically, 0.5 to 2.0 mm., subhedral, stout prismatic, twinned. Groundmass: 45 to 55 per 
cent, 0.05 to 0.2 mm., anhedral, prismatic and equant, arrangement of laths diverse. 
Labradorite, A^An-j. Phenocrysts: about i per cent, not notable megascopically, 0.5 
to 2.omm., subhedral, stout prismatic, sometimes clustered, twinned. Groundmass: about 
10 to 15 per cent, 0.03 to o. i mm., anhedral, prismatic, arrangement diverse. 
Leucite. Phenocrysts: about 5 to 10 per cent, 0.5 to 5.0 mm., subhedral to anhedral, 
equant, few inclusions. Groundmass: about 5 to 10 per cent, o.i to 0.5 mm., anhedral, 
equant, round sections, few inclusions. 
Augite. Phenocrysts: about 3 per cent, 0.2 to 2.0 mm., subhedral to anhedral, pris- 
matic and equant, often fragmentary, pale greenish-gray or colorless. Groundmass: about 7 per 
cent, 0.02 to 0.05 mm., anhedral, prismatic and equant, colorless or very pale gray. 
