There are, however, two minerals which up to the present time have been found in association only with the older and newer rock-masses respectively. These are _muscovite_, or the white form of mica, which occurs in so many granites, but has not yet been discovered in any modern representative of that rock; and leucite, which is not yet known in rocks of older date than the Tertiary. When we remember that muscovite would appear to be a product of deep-seated igneous action, and is only found in rock-masses that have been formed under such conditions, we shall be the less surprised at its non-occurrence in rocks of recent date, especially if we bear in mind the fact that very few of the younger granitic rocks have as yet been exposed at the surface by denudation. With respect to leucite, on the other hand, it must be remembered that it is a very unstable mineral which appears to be easily changed into felspar. It is by no means improbable, therefore, that some ancient igneous rocks which now contain felspar were originally leucitic rocks. To the view that the action of volcanic forces upon the globe during past geological times was similar in kind to that which we now observe going on around us, still another objection has been raised. It has been asserted that some of the deposits of igneous rock associated with the older geological formations are of such a nature that they could not possibly have been accumulated around volcanic vents of the kind which we see in operation around us. [Sidenote: VOLCANIC ORIGIN OF ANCIENT IGNEOUS ROCKS.] Mr. Mallet has declared that the igneous products of the Palæozoic period differ fundamentally in character from those materials formed by volcanic action during the later Secondary and the Tertiary periods. Upon what observations these generalisations are based he has given us no information, and the enormous mass of facts which have been collected in recent years concerning the structure of the lavas and fragmental volcanic deposits of the pre-Cambrian, Cambrian, Silurian, Devonian and Carboniferous periods, all point to a directly opposite conclusion. The more carefully we carry on our investigations concerning these ancient lavas, by the aid of chemical analysis and microscopic study, the more are we convinced of the essential identity of the ancient and modern volcanic rocks, both in their composition and their minute structure. Of great masses of dust produced by crushing, such as Mr. Mallet has supposed to have been formed during the earlier geological periods, there is not the smallest evidence; but we everywhere find proofs, when the rocks are minutely examined, of the vesicular structure so characteristic of materials produced by explosive volcanic action.