Another piece of evidence pointing in the same direction, is derived from those great fissures communicating with the interior of our globe which become filled with metallic minerals, and are known to us as mineral-veins. In these mineral-veins the native metals, their alloys, and combinations of these with sulphur, chlorine, phosphorus, &c., are frequently present. But oxides of the metals, except as products of subsequent alteration, occur far less frequently than in the earth's crust generally. Hence we are led to conclude that the substances which in the outer part of the earth's crust always exist in combination with oxygen, are at greater depths in a free and uncombined condition. Nor is it a circumstance altogether unworthy of attention that the researches of Mr. Norman Lockyer and other astronomers, based on the known facts of the relative densities of the several members of the solar system, and the ascertained relations of the different solar envelopes, have led to conclusions closely in accord with those arrived at by geologists. These researches appear to warrant the hypothesis that the interior of our globe consists of metallic substances uncombined with oxygen, and that among these metallic substances iron plays an important part. Our globe, as we know, is a great magnet, and the remarkable phenomena of terrestrial magnetism may also not improbably find their explanation in the fact that metallic iron forms 80 large a portion of the earth's interior. The interesting facts which we have been considering may be made clearer by the accompanying diagram (fig. 88). The materials ejected from volcanic vents (lavas) are in almost all cases compounds of silicon and the various metals with oxygen. In the lighter or acid lavas oxygen constitutes one-half of their weight, and the proportion of metals of the iron-group is very small. As we pass to the heavier intermediate and basic lavas, we find the proportion of oxygen diminishing, and the metals of the alkaline earths (magnesium and calcium) with the metals of the iron-group increasing, in quantity. In the small and interesting group of the ultra-basic lavas the proportion of oxygen is comparatively small, and the proportion of magnesium and iron very high. So much for the terrestrial rocks. [Illustration: Fig. 88.--Diagram illustrating the relation between the Terrestrial and the Extra-Terrestrial Rock.] [Sidenote: TERRESTRIAL AND EXTRA-TERRESTRIAL ROCKS.] Now let us turn our attention to the extra-terrestrial rocks or those found in meteorites. The Asiderites are quite identical in composition with the ultra-basic lavas of our globe, but in the Sporadosiderites and the Syssiderites we find the proportion of oxygen rapidly diminishing, and that of metallic iron increasing. Finally, in the Holosiderites the oxygen entirely disappears, and the whole mass becomes metallic.