Metamorphic Rocks. 41 



the other, then, by intense and long-continued pressure, 

 heat, and chemical changes that took place in conse- 

 quence of infiltrations among the strata themselves, by 

 degrees they became changed into hard masses, consisting 

 of shale, sandstone, conglomerate, or limestone, as the 

 case may be. But these have not always remained in 

 the condition in which they were originally consoli- 

 dated, for it has often happened that disturbances of a 

 powerful kind took place, and strata originally flat have 

 been bent into every possible curve. 



For long it was the fashion to attribute most of the 

 disturbances that the outer part of the earth has under- 

 gone to the intrusion of igneous rocks. The inclined 

 positions of beds, the contortions of stratified formations 

 in mountain chains, and even the existence of impor- 

 tant faults in fact, disturbance of strata generally 

 were apt to be referred to direct igneous action opera- 

 ting from below. Granite and its allies, from the time 

 of Hutton, were always, without exception, included in 

 the ordinary list of igneous rocks, and some writers of 

 deserved reputation still do so. In connection with 

 this subject, gneiss, and other kinds of metamorphic 

 rocks were, and by some are still, supposed to be exclu- 

 sively the effect of the direct intrusion of granite among 

 previously unaltered strata. 



As a general rule highly metamorphosed rocks 

 occur in regions where the strata have been greatly 

 disturbed. Such rocks, when the metamorphism is 

 extreme, consist of gneiss, which may be micaceous, 

 hornblendic, or chloritic ; and of mica-schist, chlorite- 

 slate, talc-slate, hornblende-rock, crystalline limestone , 

 quartz-rock, and a number of others, which it is not 

 necessary for my present purpose to name. In Scotland, 

 Irelaod, Norway, Canada, &c., limestones, calcareous 



