IV.—Notes on Progressive Metamorphism

Extract

On no branch of geology has there been more imaginative writing than upon metamorphism. Some speculations have, indeed diverged into the humorous. Keen-eyed observers, with vision capable of piercing through miles of solid rock, have described to us those processes of subterranean cookery by which Pluto has converted sandstones and shales into granite, metamorphic schists and porphyries. If water was used, the rocks were “stewed”; if not they were only “baked” or perhaps “melted.” Furthermore, it was declared, and text-books submissively taught, that rocks of almost every epoch have sometimes undergone these mystic changes: In North America, some metamorphic schists were “Triassic” while in the European Alps we had granite and gneiss of “Eocene” age. In Britain, men were more moderate, and we were not asked to extend our faith beyond the limits of the Palæozoic formations. The evidence on which these conclusions were advanced was mainly of two kinds. A metamorphic rock associated with unaltered beds was proved to be of the same age as the latter, because either it overlay or passed into them. Thus, as an illustration of the former, the newer gneissic rocks of the Northern Highlands were said to be Ordovician, because they rested on Ordovician limestone and quartzite. Murchison saw the gneiss lying on the top of the limestone or the quartzite, and he said that the upper beds were the younger. Everybody believed him, except Nicol, who died discredited and disheartened. Now, we know that Nicol was right, and the great authority, misled by superficial appearances, was wrong.