Parr II. § iv.] MICROSCOPIC CHARACTERS OF ROCKS, 99 
that these minerals, even the refractory quartz, have undoubtedly 
- erystallized out of molten solutions. 
In inclusions of a truly vitreous nature traces of devitrification 
may not infrequently be seen. In particular microscopic crystallites 
(p. 100) make their appearance, like those in the ground-mass of 
the rock. Sometimes the inclusions,-like the general ground-mass, 
have an entirely stony character. This may be well observed in 
those inclusions which have not been entirely separated from the 
surrounding ground-mass, but are connected with it by a narrow neck 
at the periphery of the enclosing crystal. In some granites and in 
elvans the quartz by irregular contraction, while still in a plastic 
state, appears to have drawn into its substance portions of the 
surrounding already lithoid base ;* but this appearance may sometimes 
be due to irregular corrosion of the crystals by the magma.” 
6. Crystals and crystalline bodies——Many component 
minerals of rocks contain other minerals (Fig.5). These occur some- 
times as perfect crystals, more usually as what are termed microliths 
(p. 101). Like the glass-inclusions, they tend to range themselves in 
lines along the successive zones of growth in the enclosing mineral. 
Such microliths are of frequent occurrence in leucite, garnet, 
augite, hornblende, calcite, fluorite, &c. It is important to observe 
that the relative order of fusibility is not always followed in the 
microliths and enveloping crystals. ‘Thus microliths of the easily 
fusible augite are in the Vesuvian lavas enclosed within the 
extremely refractory leucite. 
e Filaments, streaks, patches, discolorations.— 
Besides the enclosures already enumerated, crystals likewise fre- 
quently enclose irregular portions of mineral matter, due to alter- 
ation of the original substance of the minerals or rocks. Thus 
‘tufts and vermicular aggregates of certain green ferruginous silicates 
are of common occurrence among the crystals and cavities of old 
pyroxenic volcanic rocks. Orthoclase crystals are often mottled with: 
patches of a granular nature due to partial conversion of the mineral 
into kaolin. The magnetite, so frequently enclosed within minerals,. 
is abundantly oxidized,and has given rise to brown and yellow patches 
and discolorations. Care must be taken not to confound these 
results of infiltrating water with the original characters of a rock. 
Practice will give the student confidence in distinguishing. them, if 
he familiarises his eye with decomposition products by studying 
slices of the weathered parts of rocks. 
B. Guass.—LEven to the unassisted eye, many volcanic rocks con- 
sist obviously in whole or in great measure of glass. ‘This substance 
in mass is usually black or dark green, but when examined in thin 
sections under the microscope, it presents for the most part a pale 
brown tint, or is nearly colourless. In its purest condition it is quite 
structureless, that is, it contains no crystals, crystallites, or other 
- distinguishable individualized bodies. But even in this state it may 
1 J. A, Phillips, Q. J. Geol. Soc. xxxi. p. 338. 2 Fouqué et Lévy, op. ctt. 
H 2 
