The careful consideration of all the facts of the case leads to the conclusion that where pumice, obsidian, and rhyolite are now being ejected at the surface, the materials which form these substances are, at various depths in the earth's interior, slowly consolidating in the form of quartz-felsite, granite-porphyry and granite. It may be that we can nowhere point to the example of a mass of rock which can be traced from subterranean regions to the surface, and is, under such conditions, actually seen to pass from the dense and crystalline condition of granite to the vesicular and glassy form of pumice; but great granitic masses often exhibit a more coarsely crystalline condition in their interior, and the offshoots and dykes which they give off not infrequently assume the form of quartz-felsite; while, on the other hand, the more slowly consolidated rocks found in the interior of some rhyolite masses are not distinguishable in any way from some of the true quartz-felsites. That which is true of the lavas of acid composition is equally true of the lavas of intermediate and basic character. The andesites, the trachytes, the phonolites, and the basalts have all their exact representatives among the plutonic rocks, and these have a perfectly crystalline or granitic structure. The plutonic and the volcanic representatives of each of these groups are identical in their chemical composition, and numerous intermediate gradations can be found between the most completely granitic and the most perfectly vitreous or glassy types. In illustration of this fact, we may again refer to the series of microscopic sections of rocks given in the frontispiece. Another objection to the conclusion that the volcanic products of earlier periods of the earth's history were identical in character with those which are being ejected at the present day is based on the fact of the supposed non-existence of the scoriaceous and glassy materials which abound in the neighbourhood of the active volcanic vents. Where, it is asked, do we find among the older rocks of the globe the heaps of lapilli, dust, and scoriæ, with the glassy and pumiceous rocks that now occur so abundantly in all volcanic districts?