M. Daubrée has proposed a very convenient classification for meteorites, dividing them into the following four groups:-- I. _Holosiderites_; consisting almost entirely of metallic iron, or of iron alloyed with nickel, stony matter being absent; but sulphides, phosphides, and carbides of several metals are often diffused through the mass. The polished surfaces of these meteoric irons, when etched with acid, often exhibit a remarkable crystalline structure. II. _Syssiderites_; in which a network of metallic iron encloses a number of granular masses of stony materials. III. _Sporadosiderites_; which consist of a mass of stony materials, through which particles of metallic iron are disseminated. IV. _Asiderites_; containing no metallic iron, but consisting entirely of stony materials. There are, besides the meteorites belonging to these principal groups, a few of peculiar and exceptional composition, which we need not notice further for our present purpose. From the above classification it will be seen that most meteorites consist of a mixture in varying proportions of metallic and stony materials. Sometimes the metallic constituents are present in greater proportions than the stony, at other times the stony materials predominate, while occasionally one or other of these elements may be wholly wanting. The stony portions of meteorites, upon careful examination, prove to be built up of certain minerals, agreeing in their chemical composition and their crystalline forms with those which occur in the rocks of the earth's crust. Among the ordinary terrestrial minerals occurring in the stony portions of meteorites, we may especially mention olivine, enstatite, augite, anorthite, chromite, magnetite, and pyrrhotite. [Sidenote: METEORITES AND ULTRA-BASIC ROCKS.] The minerals which occur in meteorites are in every case such as are found in the more basic volcanic rocks--quartz, and the acid felspars, with the other minerals which occur in acid rocks, being entirely absent in the 'extra-terrestrial' rocks. Now, besides the three great classes of lavas which we have described as being ejected from volcanic vents, there are some rarer materials occasionally brought from the earth's interior by the same agency, that present a most wonderful resemblance to the stony portions of meteorites. These materials we may call 'ultra-basic rocks.' Their specific gravity is very high, usually exceeding 3, and they contain a very low percentage of silica; on the other hand, the proportion of iron and magnesia is often much greater than in ordinary terrestrial rocks. But the most remarkable fact about these ultra-basic rocks is, that they are almost entirely composed of the minerals which occur in meteorites; namely, olivine, enstatite, augite, anorthite, magnetite, and chromite.