3 o8 THE POPULAR SCIENCE MONTHLY. 



erals 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 ("Physi- 

 cal 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 min- 

 erals 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 inclu- 

 sion in granite of blocks of slate which have been transported some 

 distance from their place of origin, supplies, however, conclusive evi- 

 dence 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 metamor- 

 phism. Mr. Walling, supposing, with others, that sediments accumu- 

 lating 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 metamor- 

 phosed 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 compen- 

 sate 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 metamor- 

 phism 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. 



