Part 16 (1/2)

The reply, therefore, to the question with which we set out is very plain; and is to the effect that the present epoch is one of comparatively low volcanic activity. The further question suggests itself, whether the volcanic phenomena of the middle Tertiary period bear any comparison with those of past geological times. This, though a question of great interest, is one which is far too large to be discussed here; and it is doubtful if we have materials available upon which to base a conclusion. But it may be stated with some confidence, in general terms, that the history of the earth appears to show that, throughout all geological time, our world has been the theatre of intermittent geological activity, periods of rest succeeding those of action; and if we are to draw a conclusion regarding the present and future, it would be that, owing to the lower rate of secular cooling of the crust, volcanic action ought to become less powerful as the world grows older.

[1] Admirably ill.u.s.trated in Prof. J. Milne's recently published work, _The Great Earthquake of j.a.pan, 1891_.

APPENDIX.

A BRIEF ACCOUNT OF THE PRINc.i.p.aL 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 cla.s.ses, 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 s.p.a.ce 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 uber 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 t.i.tano-ferrite (t.i.taniferous magnet.i.te). Olivine is often present; and 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 gla.s.s paste, or ”ground-ma.s.s,” 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, apat.i.te, and magnet.i.te.[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.