
44 STRUCTURAL AND FIELD GEOLOGY 
Under the microscope what seems to be a thoroughly compact 
groundmass is sometimes resolved into an intimate aggregate of lath- 
shaped microlites of sanidine, often fluidally arranged, entangled among 
which are frequently seen crystalline granules of plagioclase, quartz, 
zircon, magnetite, apatite, etc. In other cases, however, the apparently 
compact groundmass is found to be composed largely of glass or of 
cryptocrystalline matter, or both, and usually exhibits perlitic, spherulitic, 
and fluxion structures. Occasionally the phenocrysts are so large and so 
abundant that in hand-specimens little or no groundmass can be seen, and 
the rock assumes a granitoid aspect. In other cases, when phenocrysts 
are sparingly present or wanting, the mass has a porcellanous or enamel- 
like appearance, with a somewhat waxy lustre. All these varieties of 
texture and structure may occur in one and the same lava-flow—lenticular 
streaks, laminz, and layers of coarser and finer grained, of lighter and 
darker materials alternating. Frequently the rock exhibits a finely 
porous or cellular structure, and occasionally spherical, flattened, and 
irregular shaped cavities appear, which may be encrusted or filled with 
quartz, opal, jasper, chalcedony, etc. Probably these siliceous minerals 
were deposited by chemical processes before the rock had completely 
cooled and solidified. e/s¢fe is the name given to a fine-grained to 
compact rock, throughout which are scattered phenocrysts of orthoclase, 
plagioclase, quartz, and ferromagnesian minerals. It is simply a more 
or less altered rhyolite, quartz-porphyry, or vitreous rock. Under the 
microscope it shows either a microcrystalline or cryptocrystalline texture. 
Spherulitic and perlitic structures are often present. 
Rhyolites have a somewhat wide distribution. The freshest kinds are 
of Tertiary age, and are sparingly represented in the British Islands ; they 
occur chiefly in Ireland (Antrim, co. Down); but the altered kinds are 
common among our Palzeozoic rocks, occurring both lava-form and 
intrusive. 
Pitchstone and Obsidian.—These represent the vitreous 
condition of acid rocks—they are hardly, therefore, indepen- 
dent rock-species, for they very often occur as the superficial 
crusts of hemicrystalline acid rocks. They contain some 
73 per cent. of silica. Pitchstone is usually dark green or 
black, but lighter green, red, brown, yellow, and even white 
varieties occur. The lustre of the rock is pitch-like or 
resinous ; the fracture usually conchoidal, but often irregular 
or splintery. Sometimes it contains very few crystallites or 
microlites—at other times it is crowded with such inclusions 
— the microlites occasionally forming  skeleton-crystals, 
as in the well-known Arran pitchstone, where they are 
feather-like and dendritic (see Plate VII. 1). Phenocrysts 
now and again abound; they are commonly either quartz 


