e of Japan, 1891_.

APPENDIX.

A BRIEF ACCOUNT OF THE PRINCIPAL VARIETIES OF VOLCANIC ROCKS.

The text-books on this subject are so numerous and accessible, that a
very brief account of the volcanic rocks is all that need be given here
for the purposes of reference by readers not familiar with petrological
details.

Let it be observed, in the first place, that there is no hard and fast
line between the varieties of igneous and volcanic rocks. In this as in
other parts of creation--_natura nil facit per saltum_; there are
gradations from one variety to the other. At the same time a systematic
arrangement is not only desirable, but necessary; and the most important
basis of arrangement is that founded on the proportion of _silica_ (or
quartz) in the various rocks, as first demonstrated by Durocher and
Bunsen, who showed that silica plays the same part in the inorganic
kingdom that carbon does in the organic. Upon this hypothesis, which is
a very useful one to work with, these authors separated all igneous and
volcanic rocks into two classes, viz., the Basic and the Acid; the
former containing from 45-58 per cent., the latter 62-78 per cent. of
that mineral. But there are a few intermediate varieties which serve to
bridge over the space between the Basic and Acid Groups. The following
is a generalised arrangement of the most important rocks under the above
heads:--

_Tabular View of Chief Igneous and Volcanic Rocks._

BASIC GROUP.

1. Basalt and Dolerite.
2. Gabbro.
3. Diorite.
4. Diabase and Melaphyre.
5. Porphyrite.

INTERMEDIATE GROUP.

6. Syenite.
7. Mica-trap, or Lampophyre.
8. Andesite.

ACID GROUP.

9. Trachyte, Domite, and Phonolite.
10. Rhyolite and Obsidian.
11. Granophyre.
12. Granite.

In the above grouping, and in the following definitions, I have not been
able to follow any special authority. But the most serviceable
text-books are those of Mr. Frank Rutley, _Study of Rocks_, and Dr.
Hatch, _Petrology_; also H. Rosenbusch, _Mikroskopische Physiographie
der Mineralien_, and F. Zirkel's _Untersuchungen ueber mikroskopische
Structur der Basaltgesteine_. We shall consider these in the order above
indicated:--

1. BASALT.--The most extensively distributed of all volcanic rocks. It
is a dense, dark rock of high specific gravity (2.4-2.8), consisting of
plagioclase felspar (Labradorite or anorthite), augite, and
titano-ferrite (titaniferous magnetite). Olivine is often present; and