50 -THE ROMAN COMAGMATIC REGION. 
Haiiyne. Groundmass, about i per cent, o. i to 0.2 mm., subhedral to anhedral, equant 
to irregular, colorless or blue, dusty inclusions. 
Magnetite. Groundmass: about 3 per cent, 0.02 mm., anhedral, equant. 
Apatite and titanite. Groundmass: less than i per cent, very small, subhedral, usual forms. 
Chemical composition as on p. 47. 
Type specimen from below Poggio Muratella, northwest shore of Lake Bracciano, Saba- 
tinian District. 
II 1. 7. 1. 3. Tavolatal Janeirose-Appianose [Leucite-Tephrite, Tavolato Type, 
Tavolatite]. 
Megascopic characters. Rocks of this type are light gray and very coarsely 
porphyritic. Large white or very pale-gray leucite phenocrysts are quite abundant. 
These vary in size from 10 to 30 mm., are for the most part euhedral with well- 
formed crystal planes, though often in fragments, and carry few evident inclusions. 
Small blue specks of haiiyne and fewer small black prisms of augite are numerous, 
and an occasional yellow grain of garnet is visible. The groundmass is light gray 
and quite aphariitic. 
Microscopic characters. The large leucite phenocrysts offer few features of 
special interest in the thin sections. They show the usual twinned structure, and 
carry only a few inclusions of blue haiiyne and less often of augite. The haiiynes 
are sometimes euhedral in cubes and dodecahedra, but more often are "corroded" 
deeply and irregularly. They are of a rather pale blue color and are often very 
dusty with the common minute inclusions, which are sometimes arranged in straight 
lines, parallel to the crystal faces and crossing each other reticulately, and again are 
irregularly scattered through the crystal and not zonally arranged. The small 
augite phenocrysts, which form anhedral stout prismoids, are usually of a deep 
brownish-green, somewhat pleochroic, and evidently contain some of the aegirite 
molecule. 
Small, round, anhedral microphenocrysts of leucite are very abundant in the 
groundmass, their outlines being rather ill-defined in the surrounding cement. Small 
microphenocrysts of feldspar are rare, in the form of anhedral, stout prismoids. 
These are partly of orthoclase and somewhat less frequently of soda-lime feldspar, 
which varies in composition from labradorite, Ab I An 2 ,to anorthite. There are also 
a few small brown garnets, generally in well-formed crystals, which frequently show 
a zonal structure, and an occasional small brown biotite table is seen. 
The micro-groundmass in which these lie contains many very small, slender 
feldspar prismoids, with a diverse arrangement, and which are in part orthoclase 
and to a much less extent labradorite. With these are small prismatic anhedra of 
the green segirite-augite. These are cemented by a colorless, feebly doubly refract- 
ing substance, which is undoubtedly nephelite. It is noteworthy that magnetite 
was wholly absent from the specimen analyzed and described. Glass is mentioned 
by Struever and others as replacing the nephelite base, but none of the specimens 
which I collected contain it. 
