132 



LYELL'S ELEMENTS OF GEOLOGY. 



Metamorphic Rocks Gneiss. 



occupying, for example, nearly the whole of Norway and Swe- 

 den, where, as in Brazil, they appear alike in the lower and 

 higher grounds. In Great Britain those members of the series 

 which approach most nearly to granite in their composition, as 

 gneiss, mica-schist and hornblende-schist, are confined to the 

 country north of the rivers Forth and Clyde. 



Many attempts have been made to trace a general order of 

 succession or superposition in the members of this family ; gneiss, 

 for example, having been often supposed to hold invariably a 

 lower geological position than mica-schist. But although such 

 an order may prevail throughout limited districts, it is by no 

 means universal, nor even general throughout the globe. To 

 this subject, however, we shall again revert in the second part of 

 this volume, when the chronological relations of the metamor- 

 phic rocks are pointed out. 



The following may be enumerated as the principal members 

 of the metamorphic class, gneiss, mica-schist, hornblende-schist, 

 clay-slate, chlorite-schist, hypogene or metamorphic limestone, 

 and certain kinds of quartz rock or quartzite. 



Gneiss. — The first of these, gneiss, may be called stratified 

 granite, being formed of the same materials as granite, namely 

 felspar, quartz, and mica. In the specimen here figured, the 



Fig. 122. 



Fragment of gneiss, natural size, section at right angles to planes 

 of stratification. 



white layers consist almost exclusively of granular felspar, with 

 here and there a speck of mica and grain of quartz. The dark 

 layers are composed of grey quartz and black mica, with oc- 

 casionally a grain of felspar intermixed. The rock splits most 

 easily in the plane of these darker layers, and the surface thus 

 exposed is almost entirely covered with shining spangles of mica. 

 The accompanying quartz however greatly predominates in 

 quantity, but the most ready cleavage is determined by the 

 abundance of mica in certain parts of the dark layer. 



Instead of these thin laminfc, gneiss is sometimes simply 



