It is an admitted fact that among the older geological formations, we much more frequently find intrusions of granitic rocks than in the case of younger ones. It is equally true that among the sediments formed during the most recent geological periods, no true granitic rocks have been detected. But if, as we insist is the case, granitic rocks can only be formed at a great depth from the surface, the £acts we have described are only just what we might expect to present themselves under the circumstances. The older a mass of granitic rock, the greater chance there is that the denuding forces operating upon the overlying masses, will have had an opportunity of so far removing the latter as to expose the underlying crystalline rocks at the surface. And, on the other hand, the younger crystalline rocks are still, for the most part, buried under such enormous thicknesses of superincumbent materials that it is hopeless for us to search for them. Nevertheless, it does occasionally happen that, where the work of denudation has been exceptionally rapid in its action, such crystalline rocks formed during a comparatively recent geological period, are exposed at the surface. This is the case in the Western Isles of Scotland and in the Pyrenees, where masses of granite and other highly crystalline rocks are found which were evidently formed during the Tertiary period. The granites which were formed in Tertiary times present no essential points of difference from those which had their origin during the earlier periods of the earth's history. The former, like the latter, consist of a mass of crystals with no imperfectly crystalline base or groundmass between them; and these crystals include numerous cavities containing liquids. Between the granites and the quartz-felsites every possible gradation may be found, so that it is impossible to say where the one group ends and the other begins; indeed, many of the rocks called 'granite-porphyries' have about equal claims to be placed in either class. Nor is the distinction between the quartz-felsites and rhyolites any more strongly marked than that between the former class of rocks and the granites; some of the more crystalline rhyolites of Hungary being quite undistinguishable, in their chemical composition, their mineralogical constitution, and their microscopic characters, from the quartz-felsites. The more crystalline rhyolites are in turn found passing by insensible gradations into the glassy varieties and finally into obsidian. [Sidenote: RELATIONS BETWEEN GRANITE AND PUMICE.]