Page:Popular Science Monthly Volume 15.djvu/322

308 indicates force; the inclosure of blocks of foreign material, when these blocks are masses of metamorphic schists, indicates plasticity, but affords no test of temperature. Where the included blocks are limestone, as in the granite of the Pyrenees described by Green ("Physical Geology," p. 322), and these blocks are externally metamorphosed, internally unchanged, we have a record of softening and heat, but not the heat of fusion. The coarser crystallization of granite means that it has been more or longer softened, so that its component minerals were free to crystallize out; but no distortion or dislocation of these minerals affords stronger proof of internal movement than is furnished by the associated schists, in which the fossils often not wholly obliterated are quite as much distorted as any minerals in the granite. The inclusion in granite of blocks of slate which have been transported some distance from their place of origin, supplies, however, conclusive evidence of movement which may even be called a flow. Cases of this kind, which are of great interest and significance, were observed by Mr. Newton in the Black Hills. The granite core of this mountain-chain incloses large angular blocks of metamorphic slate which have been torn from their connections and carried bodily upward. The granite also shows a kind of slickenside-jointing, which proves that when in a plastic but not fused condition it was squeezed out of a fissure or opening in the harder overlying schists. We have here proof that the granite in its lower position has been more softened than the schists, and that it has been more moved; but we have no proof that it has been subjected to greater pressure.

In contrast with the theory of Mr. King, that granites have been produced by great pressure, is that promulgated by Mr. H. F. Walling (Proceedings of the American Association at the St. Louis meeting), in which lateral pressure is practically ignored as a cause of metamorphism. Mr. Walling, supposing, with others, that sediments accumulating along shores have sunk by their gravity, recognizing the fact that the static equilibrium must be maintained by the rising of the areas lightened by erosion in the removal of these sediments, attributes mountain elevation to this ascent, and the corrugation of metamorphosed rocks to the lateral flow of material from the sinking to the rising areas. There is certainly great force in the reasoning used by Mr. Walling to show that there must be rising as well as sinking areas, and a subterranean flow of matter from one to the other to compensate for the transfer of eroded material on the surface; but it seems doubtful whether the traction produced by the adhesion of the solid strata above to the moving mass below could produce slaty cleavage and other phenomena, which we have been accustomed to attribute to the lateral thrust produced by the crushing down of the rigid crust on a shrinking nucleus. It is hardly necessary to say that the metamorphism of granite is, according to Mr. Walling, due to the subterranean heat to which it has been exposed in its descent far below the surface.