Earthquakes are mainly due to the injection of intensely heated lava into strata of cold rock in the process of forming dikes. When a volcano pours its lava out of its summit-crater, the eruption may be wholly free from earth tremors, as is often the case on the Hawaiian Islands; and this gives rise to the popular belief that active volcanoes are in some way a safety-valve for the subterranean forces. When, however, the shrinkage of the earth’s crust or the explosive force of pent-up vapors cracks the solid rock, thus giving passage to the molten mass which must be supposed to underlie this volcanic region, the sudden contact of two bodies of very different temperatures (perhaps two thousand degrees) must cause vibrations entirely sufficient to account for the worst earthquake recorded. That the supply of molten rock is ample beneath the crust of this region, we have proof in the constant activity of Izalco, which for more than a century has poured out lava with the other ejections. This theory of earthquake action is so simple that it must commend itself to any one who has observed the powerful vibrations excited by placing a cold kettle upon a hot stove, or by admitting with force a stream of hot water into a bath-tub partly filled with cold water. It may be stated also that lava is a remarkably poor conductor of heat (I have been able to walk over a crust that bent beneath my weight, and again where I left footprints in the half-hardened lava), and solid lava might retain a temperature of less than two hundred within a few feet of a molten mass ranging among the thousands of degrees. The secular refrigeration of the subterranean molten masses due to the slight conductivity of solid lava is well illustrated in the temperature of hot-springs, that remains unchanged for centuries. Eruptions are usually of an explosive nature in the Central American region (as described in the outbreak of Coseguina), and the ejected ash is scattered often to a great distance to form by its decomposition layers of soil especially fitted for the cultivation of coffee, sugar, and the vine. Sulphur is not so abundantly deposited as at Ætna, Hekla, or even the Mexican volcanoes. APPENDIX. What an attic-room is to the thrifty housewife, an appendix is to the maker of a book. Some things that do not seem to be in place in the parlor or chamber are yet useful, and altogether too good to be thrown away, so they are put into the garret to await the expected use. In a book there are matters that the writer thinks ought to interest some reader, things that will be missed if they are not under the same roof,—I mean between the covers of the volume in hand,—and yet the skill is wanting to incorporate these odd pieces (of furniture, if you wish) in the orderly chapters of the book. And so I give you here several long notes and some longer lists. A LIST OF COMMON CABINET WOODS, DYE-WOODS, AND TIMBER.