710 DYNAMICAL GEOLOGY. 



Silica + alumina and potash, soda, or lime Feldspar (p. 55). 



" " + alkali, magnesia, and boracie acid Tourmaline (p. 58). 



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. 



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 as wide as the world, furnished abundantly with 

 soda and magnesia, and in smaller proportions with many other 

 ingredients. 



2. The attending circumstances were favorable for the escape of subterra- 

 nean heat. — The rocks during a period of metamorphism are under- 

 going extensive displacements and foldings, profound fracturings 

 and faultings, as illustrated in the examples which have been de- 

 scribed. Metamorphic rocks are always displaced and folded 

 rocks, and never for any considerable distance horizontal. Where 

 the foldings are most numerous and abrupt, reducing the strata to 

 a system of parallel dips by the pressing of fold upon fold, there, 

 as remarked by the Professors Eogers, the metamorphism is most 

 complete. In the case of mineral coal, the bitumen is more com- 

 pletely expelled the greater the disturbance of the strata ; and in 

 the metamorphic region of Khode Island the coal is changed even 

 to graphite by the heat (p. 410). 



3. Thermal springs in metamorphic regions. — The thermal springs of 

 Virginia are regarded by the Professors Eogers as owing their heat 

 to the same cause which produced the consolidation and metamor- 

 phism in the Appalachian region ; and they instance as evidence 

 the fact that the localities where they occur are generally situated 

 over the axis of some fold in the Appalachian strata. 



It appears from the above that the escape of subterranean heat 

 took place during a prolonged epoch of profound subterranean dis- 

 turbance. As the epoch slowly progressed, multiplying folds and 

 fractures, the heat as gradually welled up from below, penetrating 

 the moist and yielding beds, — in some regions, where the uplift- 

 ing was least, only solidifying the beds ; in those most disturbed, 

 crystallizing them, and filling them with veins. 



Local cases of metamorphism from hot mineral waters and dikes 

 of igneous rocks have occurred without upturnings. But these 

 cases, while the same in their chemistry, are no examples of the 

 great physical conditions under which the metamorphism of the 

 thick formations has taken place. 



