ch grains or crystals of
quartz and sanidine, with other accessory minerals, are imbedded. They
occur amongst the volcanic rocks of the British Isles, Hungary, and the
Lipari Islands, from which the name _Liparite_ has been derived.

(_e._) _Obsidian (Pitchstone)._--This is a vitreous, highly acid rock,
which has become a volcanic glass in consequence of rapid cooling,
distinct minerals not having had time to form. It has a conchoidal
fracture, various shades of colour from grey to black; and under the
microscope is seen to contain crystallites or microliths, often
beautifully arranged in stellate or feathery groups. Spherulitic
structure is not infrequent; and occasionally a few crystals of
sanidine, augite, or hornblende are to be seen imbedded in the glassy
ground-mass. The rock occurs in dykes and veins in the Western Isles of
Scotland, in Antrim, and on the borders of the Mourne Mountains, near
Newry, in Ireland.

11. GRANOPHYRE.--This term, according to Geikie, embraces the greater
portion of the acid volcanic rocks of the Inner Hebrides. They are
closely allied to the quartz-porphyries, and vary in texture from a fine
felsitic or crystalline-granular quartz-porphyry, in the ground-mass of
which porphyritic turbid felspar and quartz may generally be detected,
to a granitoid rock of medium grain, in which the component dull felspar
and clear quartz can be readily distinguished by the naked eye.
Throughout all the varieties of texture there is a strong tendency to
the development of minute irregularly-shaped cavities, inside of which
quartz or felspar has crystallised out--a feature characteristic of the
granites of Arran and of the Mourne Mountains.

12. GRANITE.--A true granite consists of a crystalline-granular rock
consisting of quartz, felspar (orthoclase), and mica; the quartz is the
paste or ground-mass in which the felspar and mica crystals are
enclosed. This is the essential distinction between a granite and a
quartz-porphyry or a granophyre. Owing to the presence of highly-heated
steam under pressure in the body of the mass when in a molten condition,
the quartz has been the last of the minerals to crystallise out, and
hence does not itself occur with the crystalline form.

True granite is not a volcanic rock, and its representatives amongst
volcanic ejecta are to be found in the granophyres, quartz-porphyries,
felsites, trachytes, and rhyolites so abundant in most volcanic
countries, and to one or oth