when abundant the rock is called "olivine-basalt." In the older rocks,
basalt has often undergone decomposition into melaphyre; and amongst the
metamorphic rocks it has been changed into diorite or hornblende rock;
the augite having been converted into hornblende.

When leucite or nepheline replaces plagioclase, the rock becomes a
leucite-basalt,[1] or nepheline-basalt. Some basalts have a glass paste,
or "ground-mass," in which the minerals are enclosed.

The lava of Vesuvius may be regarded as a variety of basalt in which
leucite replaces plagioclase, although this latter mineral is also
present. Zirkel calls it "Sanidin-leucitgestein," as both the
macroscopic and microscopic structure reveal the presence of leucite,
sanidine, plagioclase, nephiline, augite, mica, olivine, apatite, and
magnetite.[2]

_Dolerite_ does not differ essentially from basalt in composition or
structure, but is a largely crystalline-granular variety, occurring more
abundantly than basalt amongst the more ancient rocks, and the different
minerals are distinctly visible to the naked eye.

A remarkable variety of this rock occurs at Slieve Gullion in Ireland,
in which mica is so abundant as to constitute the rock a "micaceous
dolerite."

2. GABBRO.--A rather wide group of volcanic rocks with variable
composition. Essentially it is a crystalline-granular compound of
plagioclase, generally Labradorite and diallage. Sometimes the pyroxenic
mineral becomes hypersthene, giving rise to _hypersthene-gabbro_; or
when hornblende is present, to _hornblende-gabbro_; when olivine, to
_olivine-gabbro_. Magnetite is always present.

These rocks occur in the Carlingford district in Ireland, in the Lizard
district of Cornwall, the Inner Hebrides (Mull, Skye, etc.) of Scotland,
and in Saxony.

3. DIORITE.--A crystalline-granular compound of plagioclase and
hornblende with magnetite. When quartz is present it becomes (according
to the usual British acceptation) a _syenite_; but this view is
gradually giving place to the German definition of syenite, which is a
compound of orthoclase and hornblende; and it may be better to
denominate the variety as _quartz-diorite_. The diorites are abundant as
sheets and dykes amongst the older palaeozoic and metamorphic rocks, and
are sometimes exceedingly rich in magnetite. Mica, epidote, and chlorite
are also present as accessories.

The rock occurs in North Wales, Charnwood Forest, Wicklow, Galway, and
Donegal, and t