he Highlands of Scotland. There can be little doubt that
amongst the metamorphic rocks of Galway, Mayo, and Donegal the great
beds of (often columnar) diorite were originally augitic lavas, which
have since undergone transformation.

4. DIABASE.--It is very doubtful if "Diabase" ought to be regarded as a
distinct species of igneous rock, as it seems to be simply an altered
variety of basalt or dolerite, in which chlorite, a secondary
alteration-product, has been developed by the decomposition of the
pyroxene or olivine of the original rock. It is a convenient name for
use in the field when doubt occurs as to the real nature of an igneous
rock. Melaphyre is a name given to the very dark varieties of altered
augitic lavas, rich in magnetite and chlorite.

5. PORPHYRITE (or quartzless porphyry).--A basic variety of
felstone-porphyry, consisting of a felspathic base with distinct
crystals of felspar, with which there may be others of hornblende, mica,
or augite. The colour is generally red or purple, and it weathers into
red clay, in contrast to the highly acid or silicated felsites which
weather into whitish sand.

6. SYENITE.--As stated above, this name has been variously applied. Its
derivation is from Syene (Assouan) in Egypt, and the granitic rocks of
that district were called "syenites," under the supposition (now known
to be erroneous) that they differ from ordinary granites in that they
were supposed to be composed of quartz, felspar, and hornblende, instead
of quartz, felspar, and mica. From this it arose that syenite was
regarded as a variety of granite in which the mica is replaced by
hornblende, and this has generally been the British view of the
question. But the German definition is applied to an entirely different
rock, belonging to the felstone family; and according to this
classification syenite consists of a crystalline-granular compound of
orthoclase and hornblende, in which quartz may or may not be present.
From this it will be seen that, according to Zirkel, syenite is
essentially distinct from diorite in the species of its felspar.[3] It
seems desirable to adopt the German view; and as regards diorites
containing quartz as an accessory, to apply to them the name of
_quartz-diorite_, as stated above, the name syenite as used by British
geologists having arisen from a misconception.

7. MICA-TRAP (LAMPOPHYRE).--A rock, allied to the felstone family, in
which mica is an abundant and essential constitu