It has frequently been asserted that the acid igneous rocks are present in much greater quantities in connection with the older geological formations than axe the basic; while, on the other hand, the basic igneous rocks are said to have been extruded in greater abundance in the more recent geological periods. But in considering this question it must not be forgotten that, as a general rule, the basic rocks undergo decomposition and disintegration far more rapidly than do the acid rocks. In consequence of this circumstance the chance of our finding their recognisable representatives among the older formations, is much less in the case of the former class of rocks than in the latter. As a matter of fact, however, we do find great masses of gabbro, diabase, and melaphyre associated even with the oldest geological formations, while trachytes and rhyolites abound in many volcanic districts where active vents exist at the present day. Upon a general review of the subject, it may well be doubted whether the supposed preponderance of acid igneous materials in the earlier periods of the earth's history, and of basic igneous materials during the later periods, rests on any substantial basis of observation. [Sidenote: AUGITIC AND HORNBLENDIC ROCKS.] Another difference which has frequently been relied upon, as distinguishing the older igneous rocks from those of more recent date, is the supposed fact that the former are characterised by the presence of hornblende, the latter by the presence of augite. It may be admitted that this distinction is a real one, but its significance and value are greatly diminished when we remember the relations which exist between the two minerals in question. Hornblende and augite are interesting examples of a dimorphous substance; in chemical composition they are identical, or rather they are liable to variation between the same limits, but in their crystalline forms and optical characters they differ from one another. It has been proved that hornblende is the stable, and augite the unstable condition of the substance in question. If hornblende be fused and allowed to cool, it crystallises in the form of augite. On the other hand, augite-crystals in rocks of ancient date are found undergoing gradual change and passing into hornblende. The mineral uralite has the outward form of augite, but the cleavage and optical properties of hornblende; and there are not wanting many facts pointing to the conclusion that rocks which now contain hornblende were originally augitic masses, in which the unstable mineral in their midst has been gradually converted into the stable one.