From the Holosiderites at one end of the chain to the add lavas at the other, we find there is a complete and continuous series; the rocks of terrestrial origin overlapping, in their least oxydized representatives, the most highly oxydized representatives of the extra-terrestrial rocks. But the discovery at Ovifak of the iron-masses, and the basalts with iron disseminated, has afforded another very important link, placing the terrestrial and extra-terrestrial rocks in closer relations with one another. All these facts appear to point to the conclusion that the earth's interior consists of metallic substances either quite uncombined or simply alloyed with one another, and among these iron is very conspicuous by its abundance. The outer crust, which is probably of no great thickness, contains an enormous proportion of oxygen and silicon combined with the materials which constitute the interior portions of our globe. It may be, as has been suggested by astronomers, that our earth consisted at one time of a solid metallic mass surrounded by a vaporous envelope of metalloids, and that the whole of the latter, with the exception of the constituents of the atmosphere and ocean, have gradually entered into combination with the metals of the nucleus to form the existing crust of the globe. But of this period the geologist can take no cognisance. The records which he studies evidently commenced at a long subsequent period, when the conditions prevailing at the earth's surface differed but little, if at all, from those which exist at the present day. Equally little has the geologist to do with speculations concerning a far distant future when, as some philosophers have suggested, the work of combination of the waters and atmosphere of the earth's surface with the metallic substances of its interior shall be completed, and our globe, entirely deprived of its fluid envelopes, reduced to the condition in which we find our satellite, the moon. * * * * * [Sidenote: PHYSICAL CONDITION OF EARTH'S INTERIOR.] There is another class of enquiries concerning the earth's interior to which the attention of both geologists and astronomers has long been directed--that, namely, which deals with the problem of the _physical condition_ of the interior of our globe.