

762 DYNAMICAL GEOLOGY. 



Silica + magnesia + lime or pvotoxyd of iron Pyroxene. 



Silica -f- magnesia -f- lime or protoxyd of iron Hornblende. 



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



Silica + alumina Andalusite. 



Silica + alumina Cyanite. 



Silica + alumina + fluorine Topaz. 



Silica + alumina + oxyds of iron Staurolite. 



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



Silica + alumina + lime and soda Scapolite. 



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



Silcia -J- alumina + oxyd of iron Epidote. 



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



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



The presence of phosphoric acid, from organic remains, has often determined the for- 

 mation in metamorphic limestones, and even sometimes in granite and other metamor- 

 phic rocks, of crystals of apatite (phosphate of lime); and the presence of fluorine may 

 have led to the crystallization of chondrodite, topaz, and some other species. 



4. Origin oi" the Heat causing Metamorphism. 



Rocks, during the process of metamorphism, are undergoing ex- 

 tensive displacements and foldings, profound fracturings and faultings, 

 if not also crushings, in some parts — as illustrated in the examples 

 which have been described on page 213. Metamorphic rocks are 

 always displaced and folded rocks, and seldom 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 the metamorphism is most complete. Novr, 

 the heat caused within the rocks by the friction, and whatever crushing 

 may have taken place, if not sufficient to produce fusion and volcanoes, 

 as Mallet has claimed (p. 719), may be sufficient for the feebler work 

 of metamorphism ; and if so, it is then true, as Wurtz was first to 

 announce, that the heat of metamorphism was made in the very rocks 

 that were altered by the movements to which they were subjected. 

 But this is not the source of all the heat ; for the deposits undergoing 

 simultaneous metamorphism have often had great depth — even several 

 miles, in some cases, — and, as stated on page 718, heat from the 

 earth's interior has risen into the pile of beds as fast as accumulation 

 went on above, and an important addition to the heat for the meta- 

 morphism may have thus been derived. Scrope and Babbage, who 

 were the first to appeal to this as the source of heat, regarded it as 

 alone sufficient ; and, accordingly, a very great thickness of overly- 

 ing deposits was thought necessary to produce the effects. Hence, it 

 happened that such rocks as gneiss, granite, and the like, have been 

 called Hypogene rocks, meaning rocks made at great depths. But there 

 is no evidence, in many cases, of so great thickness as the theory de- 

 mands ; and, again, a thickness of 10,000 to 15,000 feet in sedimen- 

 tary beds has been observed — as in the Nova Scotia coal formation 



