Page:South African Geology - Schwarz - 1912.djvu/61

 It is evident that nothing produced by the breaking down of the granite is lost; the materials are only scattered in the sedimentary rocks. If now, under extreme action of metamorphism, the water circulating in the pores of the rocks is allowed to carry substances from one rock to the other, the various substances may be reunited; the slates and schists will receive material from the quartzites and marbles and eventually the three types become welded into one common rock, which should be nearly the same in composition as the original granite. This is found actually to be the case, for at the end of the metamorphic series there is a class of rocks called gneisses, which are granites with banded structure containing just such quartz, felspar, and hornblende crystals as in the original granite, but laid one upon the other in layers. It is probable that there is a step further still, when the banded structure disappears and the gneiss does become a granite, and then the circle of change be- comes complete.

Igneous rocks are supposed to have come up from the interior of the earth, molten with the original heat of the earth. In the old days heat was always connected with fire — ignis — therefore the molten rocks received the name igneous rocks. If the igneous rocks were once molten with heat it was not that due to the combustion of substances in the interior of the earth. There is, however, a large body of evidence which goes to show that they are rather solutions of substances in one another. If a disk of silver be laid on one of gold, and the two be kept under a very great pressure for a considerable time, the silver will be found to have worked into the gold, and vice versa; the two have dissolved each other, given pressure and time. In a similar manner rock substances