[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.
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 über 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 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.