PETROGRAPHY. 19 
out, and will repay some study. This matter will be discussed later in connection 
with the formation of leucite. 
I. 5. 1. 4-3. Ischial Nordmarkose-Phlegrose [Augite-Trachyte, Ischia Type], 
Megascopic characters. Rocks of this type are light gray, rough in feel, and 
highly porphyritic. The most abundant phenocrysts are of feldspar, which con- 
stitute about 30 per cent of the rock. These are tabular or stout prismatic, from 
10 to 20 mm. long, colorless, and with a highly vitreous luster. From the absence 
of twinning striations they would be considered to be orthoclase. Small prismatic 
phenocrysts of black augite and grains of magnetite are very rare, not making up 
more than i or 2 per cent of the rock. The groundmass is very light gray and 
phanerocrystalline, but very fine-grained. It is obviously composed almost exclu- 
sively of salic minerals, presumably feldspars. 
Microscopic characters. The thin sections show the following minerals : Alkali 
feldspar very abundant, with less augite and magnetite, and accessory lavenite and 
sodalite. The texture is typically holocrystalline, and with a characteristic trachy- 
tic fabric. 
The feldspar phenocrysts are of soda-orthoclase in thick subhedral tables, tab- 
ular parallel to b (oio), or occasionally stoutly prismatic parallel to the a axis. They 
are not uncommonly fragmentary, having been broken during the flow. Although 
their sodic character is certain from chemical analysis, the microscopic appearances 
so characteristic of soda-orthoclase are usually absent. They show, for the most 
part, a clear, even gray between crossed nicols, without the lamellae of microcline, 
microperthitic intergrowths of orthoclase and albite, or the common moire* appear- 
ance. The rare augite phenocrysts are subhedral, in stout prismoids, and of a very 
pale-gray color, or with only an extremely faint tinge of green. Inclusions are very 
rare both in these and in the feldspar phenocrysts. 
The groundmass has the typical trachytic fabric, and usually with marked evi- 
dences of flow. It is composed in very great part of small prismoids of alkali feld- 
spar, elongated parallel to the axis a. Small prismoids of colorless augite and 
grains of magnetite are present, but only in negligible amounts. In some specimens 
lavenite occurs as a rare accessory, in small, yellow, pleochroic prismoids, often 
arranged in divergent clusters ; but this mineral can not be regarded as character- 
istic of the type. The same is true of sodalite, which is sometimes present in very 
small amount in the groundmass as colorless, rounded anhedra, as well as in crevices 
of the rock. No glass could be detected in most of the specimens, though it may be 
present to a very slight extent. 
Chemical composition. An analysis of this type was published some years ago, 
and is here repeated in more complete form. An older analysis by Fuchs is also 
given for comparison. 
The analysis is noteworthy because it shows that a typical "trachyte," which 
is regarded as composed essentially of orthoclase, may carry as much soda as potash 
