728 DYNAMICAL GEOLOGY. 



cases of the original planes of lamination is evidence that this plastic 

 or semi-fused state was not common in metamorphic operations. It 

 was one of the conditions requisite for the formation of granite, — a 

 non-schistose rock ; and the transitions from gneiss to granite, which 

 are by imperceptible gradations, indicate different degrees of this plastic 

 state. 



There may seem to be some difficulty in accounting for metamor- 

 phic results, on the ground of the diversity of mineral species that are 

 produced. But, in the first place, the elements constituting these 

 species are few in number, — silica, alumina, potash, soda, lime, mag- 

 nesia, and the oxyds of iron being all that are necessary for the great 

 majority of them. In the second place, as just stated, the material of 

 sedimentary strata is, to a large extent, nothing but pulverized meta- 

 morphic rocks, so that the metamorphism is often only a new crystalli- 

 zation of the minerals already present. In the third place, the organic 

 remains, out of which many rocks have been largely made, even the 

 arenaceous and argillaceous, have contributed a variety of ingredients, 

 besides carbonate of lime, — the most important of which are phos- 

 phoric acid and fluorine. 



The following table presents a general view of the composition of the more common 

 rock-making materials, showing their close similarity. These species are briefly de- 

 scribed on pages 52-58. The names mica and feldspar each include several species: — 



Silica . Quartz. 



Silica + magnesia and water . . . . . . . - Talc. 



Silica + magnesia and water Serpentine. 



Silica + magnesia + lime or protoxyd of iron . . . . . Pyroxene. 



Silica + magnesia + lime or protoxyd of iron .... Hornblende. 



Silica + magnesia + alumina and protoxyd of iron .... Chlorite. 



Silica + alumina Andalusite. 



Silica + alumina Cyanite. 



Silica -j- alumina + fluorine Topaz. 



Silica + alumina + oxyds of iron Staurolite. 



Silica + alumina + oxyds of iron + potash or magnesia . . Mica. 



Silica + alumina + lime and soda Scapolite. 



Silica + alumina + lime, magnesia, iron, or manganese . . Garnet. 



Silica + alumina + oxyd of iron Epidote. 



Silica + alumina + potash, soda, or lime Feldspar. 



Silica + alumina + alkali, magnesia, and boracic acid . . . Tourmaline. 



The presence of phosphoric acid, from organic remains, determines often the formation 

 in metamorphic limestones, and even sometimes in granites, of crystals of apatite 

 (phosphate of lime); and the presence of fluorine may promote the crystallization of 

 chondrodite, topaz and some other species. When the alkalies are absent from a clay 

 or shale, metamorphism, as Hunt has stated, cannot produce feldspar, but may fill the 

 slate with staurolite, andalusite, cyanite, or other non-alkaline minerals. 



Again, all heated subterranean waters would become mineral waters, 

 and would serve to carry the material they held in solution wherever 

 they might have access. In addition, the ocean is a mineral source 



