A piece of granite and a piece of pumice may at first sight appear to present so many points of difference, that it would seem quite futile to attempt to discover any connection between them. Yet, if we analyse the two substances, we may find that in ultimate chemical composition they are absolutely identical. There is nothing irrational, therefore, in the conclusion that the same materials under different conditions may assume either the characters of granite on the one hand, or of pumice on the other; the former being consolidated under circumstances in which the chemical and crystalline forces have had the freest play and have used up the whole of the materials to form crystallised minerals, while the latter has cooled down and solidified rapidly at the surface, in such a way that only incipient crystallisation has occurred, and the glassy mass has been reduced to a frothy condition by the escape of steam-bubbles from its midst This conclusion receives the strongest support from the fact that examples of every stage of the change, between the glassy condition of pumice and the crystalline condition of granite, may be detected among the materials of which the globe is built up. There is still another class of facts which may be adduced in support of the same conclusion. Many lavas, as we have seen, contain crystals of much larger dimensions than those constituting the mass of the rock, which is then said to be 'porphyritic' in structure. The porphyritically embedded crystals, when carefully examined, are often seen to be broken and injured, and to exhibit rounded edges, with other indications of having undergone transport. When examined microscopically, too, they often present the cavities containing liquids which distinguish the crystals of plutonic rocks. All the facts connected with these porphyritic lavas point to the conclusion that while the crystals in their groundmass have separated from the liquefied materials near the surface, the large embedded crystal, have been floated up from great depths within the earth's crust, where they had been originally formed. [Sidenote: GRANITIC REPRESENTATIVES OF OTHER LAVAS.]