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 illustrated in Prof. J. Milne's recently published work, _The Great Earthquake 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.