All these facts point to one conclusion--namely, that during all past geological periods, materials similar to those which are now being extruded from volcanic vents were poured out on the earth's surface by analogous agencies. If we could trace the lava-streams of the present day down to the great subterranean reservoirs from which their materials have been derived, we should doubtless find that at gradually increasing depths, where the pressure would be greater and the escape of heat from the mass slower, the rocky materials would by degrees assume more and more crystalline characters. We should thus find obsidian or rhyolite insensibly passing into quartz-felsite and finally into granite; trachyte passing into orthoclase-porphyry and syenite; and basalt passing into dolerite, augite-porphyry, and gabbro. On the other hand, if we could replace the great masses of stratified rocks which must once have overlain the granites, syenites, diorites, and gabbros, we should find that, as we approached the original surface, these igneous materials would gradually lose their crystalline characters, and when they were poured out at the surface would take the forms of rhyolite, trachyte, andesite, and basalt--all of which might occasionally assume the glassy forms known as obsidian or tachylyte. [Sidenote: ALTERED FORMS OF ANCIENT LAVAS.] But while we insist on the essential points of similarity between the lavas poured out upon the surface of the earth during earlier geological periods and those which are being extruded at the present day, we must not forget that by the action of percolating water and acid gases, the mineral constitution, the structure, and sometimes even the chemical composition of these ancient lavas may undergo a vast amount of change. In not a few cases these changes in the characters of a lava may be carried so far that the altered rock bears but little resemblance to the lava from which it was formed, and it may be found desirable to give it a new name. Among the rocks of aqueous origin we find similar differences in the materials deposited at different geological periods. Clay, shale and clay-slate have the same composition, and the two latter are evidently only altered forms of the first mentioned, yet so great is the difference in their characters that it is not only allowable, but desirable, to give them distinctive names.