The alteration of the lavas and other igneous rocks does not, however, stop with the production of the melaphyres, porphyrites, and felstones. By the further action of the water and carbonic acid of the atmosphere, the basic lavas are reduced to the soft earthy mass known as 'wacke,' and the intermediate and acid lavas to the similar material known as 'claystone.' As the passage of water and carbonic acid gas through these rock-masses goes on, they are eventually resolved into two portions, one of which is insoluble in water and the other is soluble. The insoluble portion consists principally of quartz, the crystals of which are almost unattacked by water and carbonic acid, and the hydrated silicate of alumina. All the sands and clays, which together make up more than nine-tenths of the stratified rocks of the globe, are doubtless derived, either directly or indirectly, from these insoluble materials separated during the decomposition of volcanic and plutonic rocks. The soluble materials, which consist of the carbonates, sulphates and chlorides of lime, magnesia, soda, potash, and iron, give rise to the formation of the limestones, gypsum, rock-salt, ironstones, and other stratified masses of the earth's crust. We thus see how the igneous materials of the globe, by their decomposition, famish the materials for the stratified rock-masses. The relations of the different plutonic and volcanic rocks to one another and to the materials which are derived from them are illustrated in the following table.
RELATIONS OF ALTERED TO UNALTERED LAVAS.
| Plutonic Rocks | Unaltered lavas | Altered lavas | Decomposed Rocks | |||||
| Granite | { | Quartz-felsite ('quartz-porphyry') | } | Rhyolite and Obsidian |  | Felstone | | Claystones |
| Syenite | { | Orthoclase-porphyry | } | Trachyte | ||||
| Diorite | { | Hornblende-porphyry | } | Andesite | Porphyrite | |||
| Miascite | { | Liebnerite porphyry | } | Phonolite | ? | — | ||
| Gabbro | { | Augite-porphyry and Dolerite | } | Basalt | Melaphyre | Wacke | ||
Some petrographers, indeed, have maintained the principle that rocks belonging to widely separated geological periods, even when they exhibit no essential points of difference, should nevertheless be called by distinct names. But such a system of classification is calculated rather to hinder than to advance the cause of science. If the palæontologist were to adopt the same principle and give distinct names to the same fossil, when it was found to occur in two different geological formations, we can easily understand what confusion would be occasioned, and how the comparison of the fauna and flora of the different formations would be thereby rendered impossible. But the naturalist, in his diagnosis of a species, wisely confines himself to the structure and affinities of the organism before him; and in the same way the petrographer, in giving a name to a rock, ought to be guided only by his studies of its chemical composition, its mineralogical constitution, and its structure, putting altogether out of view its geographical distribution and geological age. Only by strict attention to this principle can we hope to arrive at such comparisons of the rocks of different areas and different periods, as may serve as the basis for safe inductions.
Before leaving this question of the relation which exists between the igneous rocks of different ages, it may be well to notice several facts that have been relied upon, as proving that the several geological periods are distinguished by characteristic igneous products.
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.
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.
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.