In the same way, among the deposits of the earlier geological periods we find rocks which were doubtless originally basalts, but in which great alterations have been produced by the percolation of water through the mass. The original rock has consisted of crystals of felspar, augite, olivine, and magnetite distributed through a glassy base. But the chemical action of water and carbonic acid may have affected all the ingredients of the rock. The outward form of the felspar crystals may be retained while their substance is changed to kaolinite, various zeolites, and other minerals; the olivine maybe altered to serpentine and other analogous minerals; the magnetite changed to hydrous peroxide of iron; the augite may be changed to uralite or hornblende; and the surrounding glassy mass more or less devitrified and decomposed. The hard, dense, and black rock known as basalt has under these circumstances become a much softer, earthy-looking mass of a reddish-brown tint, and its difference from basalt is so marked that geologists have agreed to call it by another name, that of 'melaphyre.' Even in their ultimate chemical compositions the 'melaphyres' differ to some extent from the basalts, for some of the materials of the latter may have been removed in solution, and water, oxygen, and carbonic acid have been introduced to combine with the remaining ingredients. But if we carefully study, by the aid of the microscope, a large series of basalts and melaphyres, we shall find that many rocks of the former class show the first incipient traces of those changes which would reduce them to the latter class. Indeed, it is quite easy to form a perfect series from quite unaltered basalts to the most completely changed melaphyres. Hence we are justified in concluding that all the melaphyres were originally basalts, just as we infer that all oaks were once acorns. Now changes, similar to those which we have seen to take place in the case of basaltic lavas, are exhibited by the lavas of every other class, which have been exposed to the influence of the same agencies,--namely, the passage of water and acid gases. But inasmuch as the minerals composing the basic lavas are for the most part much more easily affected by such agencies than are the minerals of acid lavas, the ancient basic rocks are usually found in a much more highly altered condition than are the acid rocks of equivalent age. [Sidenote: NAMES GIVEN TO ALTERED LAVAS.]