462 SPECIAL GEOLOGY. 



of the same elements as granite (mica,. quartz, and felspar), 

 and these being arranged in undulated, or contorted layers, 

 may be considered slaty granite. The system contains, in 

 addition to beds of gneiss, mica-schist, quartz-rock, primary 

 limestone, hornblende-schist, and clay-slate, alternating in a 

 very irregular manner. If the gneiss and the mica-schist 

 are regarded as sedimentary rocks, metamorphosed and ren- 

 dered crystalline by heat, the change in the case of gneiss 

 appears to have been but partial, this rock having been 

 formed from the disintegration of granite, and deposited in 

 its present form by an ocean which was, probably, too warm 

 for the support of animal life. On the other hand, the gra- 

 dation of gneiss into mica-schist and slaty rocks, and of these 

 into others which are unquestionably of aqueous origin, 

 strengthen the probability of the whole being of the common 

 origin assigned them by Dr. Hutton, and owing their dis- 

 similar physical characters to the unequal degrees of heat 

 to which they have been exposed.* The stratification of 



"~ i saw 

 FIG. 301. Vein of Porphyry, traversing argillaceous schist, St. Agnes, Cornwall. 



gneiss is more or less distinct ; the strata are often inclined 

 to the horizon at a very great angle. Mountains composed of 

 gneiss are seldom so steep as those of granite; and their 



* Professor Phillips : Treatise on Geology, in the Cabinet Cyclopaedia. 



