The ultra-basic rocks often occur under very peculiar conditions. Sometimes they are found forming ordinary volcanic protrusions through the sedimentary rocks. The rocks named pikrites, lherzolites, dunites, &c., are examples of such igneous protrusions composed of these ultra-basic materials, and probably all the true serpentines are rocks of the same class which have absorbed water and undergone great alteration. The ultra-basic rocks sometimes contain platinum and other metals in the free or uncombined state. But not unfrequently we find among the ordinary ejections of volcanoes, nodules and fragments of such ultra-basic materials, which have clearly been carried up with the other lavas from great depths in the earth's crust. Thus in Auvergne, the Eifel, Bohemia, Styria, and many other volcanic districts, the basaltic lavas and tuffs are found to contain nodules composed of the minerals which are so highly characteristic of meteorites. Such nodules, too, often form the centres of the volcanic bombs which are thrown out of craters during eruptions. We thus see that materials identical in composition and character with the stony portions of meteorites, exist within the earth's interior, and are thrown out on its surface by volcanic action. A still more interesting discovery has been made in recent years; namely, that materials similar to the metallic portion of meteorites, and consisting of nickeliferous iron, also occur in deep-seated portions of the earth's crust, and are brought to the surface during periods of igneous activity. In the year 1870, Professor Nordenskiöld made a most important discovery at Ovifak, on the south side of the Island of Disko, off the Greenland coast. On the shore of the island a number of blocks of iron were seen, and the chemical examination of these proved that, like ordinary metallic meteorites, they consisted of iron alloyed with nickel and cobalt. [Sidenote: IRON-MASSES OF OVIFAK.] Now, when the facts concerning the masses of native iron of Ovifak were made known, the first and most natural explanation which presented itself to every mind was, that these were a number of meteorites which at some past period had fallen upon the earth's surface. Metallic iron. Opaque crystals of magnetite (black oxide of iron). Transparent crystals of felspar, augite, and olivine. [Illustration: Fig. 87.--Section of basalt from Ovifak, Greenland, with particles of metallic iron diffused through its mass.]