

34 STRUCTURAL AND FIELD GEOLOGY 
congeals to form a vitreous rock, throughout which the 
several mineral compounds exist in essentially the same 
diffused condition as in the molten magma. It will be 
understood, then, that when a molten mass cools and con- 
solidates rapidly, a glassy or vitreous rock results; with less 
rapid cooling a hemzcrystalline rock is formed; while very 
slow cooling gives rise to a holocrystalline rock. But as 
traces of crystallisation are rarely or never quite absent 
from a volcanic glass, all these types may, for purposes of 
description, be included under the head of crystalline igneous 
rocks, 
Vitreous Rocks. Their General Character.—Many of these seem to the 
unassisted eye smoothly homogeneous, and to contain no trace of crystal- 
line ingredients. When thin slices, however, are subjected to microscopic 
examination, they rarely fail to show, in less or greater abundance, 
certain minute bodies, some of which have obviously a crystalline 
structure, while others show no such structure, but may be looked upon 
as merely the embryos of crystals. Some of these forms are shown in 
Plate VII. 1. Cvystallite is the name given to the minute bodies which 
do not react on polarised light, and are apparently destitute of crystalline 
structure. As a rule crystallites afford no hint as to the nature of the 
mineral into which they might have developed had their growth not been 
arrested. Other minute bodies (J/zcrolites) which give a definite reaction 
with polarised light, show a further stage in crystal-development, and 
are often of such a character that it is possible to say to what mineral 
species they belong. Besides crystallites and microlites, more or less 
well-developed crystals of relatively large size may occur disseminated 
‘through a vitreous rock (see Plate VII. 2). 
Certain other structures of frequent occurrence in glassy rocks may be 
briefly referred to. Amongst these are small globules termed Spherulites 
(see Plates VII. 3; XIII. 1). They vary in size from a millet-seed to a 
pea, and under the microscope show an internal divergent or radiating 
fibrous structure. Similar spherical bodies, sometimes larger than hazel- 
nuts, are now and then developed in artificial glass, their internal fibrous 
structure being quite apparent to the naked eye. Not infrequently, 
glassy rocks contain small enamel-like globules, which, in thin sections 
under the microscope, often exhibit an imperfectly developed concentric 
or perlitic structure (see Plate VII. 4). Spherulites may occur sporadi- 
cally or be closely packed together ; and, similarly, perlitic structure 
may be sparsely or abundantly developed—some glassy rocks, indeed, 
appearing as if composed entirely of enamel-like globules. A vitreous 
rock having this character well marked is often termed Pev/z‘e. 
There are certain other structures which, although not confined to 
vitreous rocks, are nevertheless more or less characteristic of these. 
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