We thus see that each of the classes of modern lavas has its representative in earlier geological periods, in the form of rocks which have evidently been derived from these lavas, through alterations effected by the agency of water and acid-gases that have permeated their mass. Thus, while the basalts are represented among the ancient geological formation by the melaphyres, the andesites are represented by the porphyrites, and the trachytes and rhyolites by different varieties of felstones. And, as we can form perfect series illustrating the gradual change from basalt to melaphyre, so we can arrange other series demonstrating the passage of andesites into porphyrites, and of trachytes and rhyolites into felsites. It must be remembered, however, that these changes do not take place in anything like determinate periods of time. Occasionally we may find lavas of ancient date which have undergone surprisingly little alteration, and in other cases there occur lavas belonging to a comparatively recent period which exhibit very marked signs of change. 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. [Sidenote: RELATIONS OF ALTERED TO UNALTERED LAVAS.] Unaltered Altered Decomposed Plutonic Rocks lavas lavas Rocks