ROCK-FORMING MINERALS 13 
in fissures in crystalline schists; and in blocks ejected from 
volcanoes. The plagioclase felspars are among the most 
important rock-formers, occurring as primary constituents of 
a large number of eruptive rocks both as macroscopic and 
microscopic individuals. They have also a wide distribution 
amongst the crystalline schists. The felspars (monoclinic and 
triclinic alike), being readily weathered and decomposed, are 
met with only now and then as ingredients of derivative 
rocks; they are not uncommon, however, in greywackés— 
especially oligoclase. 
A few notes on the individual plagioclase felspars may be added :— 
Albite (scda felspar) : usually white ; resembles orthoclase, from which 
it may be distinguished by its greater specific gravity (orthoclase, 2-54 
to 2:58; albite, 2-61 to 2:64) and the character of its twinning. Although 
albite frequently appears as lamellar intergrowths in orthoclase and 
microcline (= mcroperthite), yet it cannot be described as an important 
constituent of igneous rocks. It is a common ingredient, however, of 
certain crystalline schists. Now and again it occurs as a “contact 
mineral” in argillaceous and calcareous rocks, near their junction or con- 
tact with intrusive eruptive rocks. 
Oligoclase is a common constituent of many eruptive rocks, especially 
of those in which quartz or orthoclase, or both together, occur as impor- 
tant ingredients, as in syenite and granite. It is present likewise in some 
diorites, and in many of the porphyries which are associated with plutonic 
rocks ; not infrequently also in trachytes and andesites ; and often in 
gneiss. 
Andesine is a frequent constituent of certain eruptive rocks, such as 
syenite, tonalite, andesite, dolerite, and basalt. 
Labradorite is a common constituent of basic eruptive rocks (gabbros, 
norites, basalts, diorites), and occurs in large masses in certain plutonic 
rocks, where it frequently shows a very fine play of colours, due to the 
interposition along the cleavage-planes of minute scales or platy 
inclusions. 
Bytownite occurs not uncommonly in andesites, basalts, and other 
- basic eruptive rocks. 
Anorthite (lime felspar) occurs not infrequently as a constituent of 
many basic igneous rocks, as in some diorites, gabbros, peridotites, 
basalts, and, less frequently, in certain andesites. It is also an occasional 
constituent of metamorphic rocks. Fine glassy crystals of this felspar 
occur in the drusy cavities of limestone-blocks which have been ejected 
from Vesuvius. 
THE FELSPATHOID GROUP 
The felspathoids (Leucite, Nepheline, Sodalite, Haiiyne, and 
