141 



CHAP. XXVI. 



Gneiss. Compact Felspar. 



GNEISS is one of the most extensive rocks in nature, 

 and if not always the lowest of the stratified ones, as 

 generally esteemed, it most frequently occupies that 

 position. It presents considerable differences in its 

 geological features and connexions ; as well as a re- 

 markable diversity of composition and structure. Under 

 the common definition, it is limited to a foliated mix- 

 ture of mica, quartz, and felspar; but this does not in- 

 clude all the substances which appertain to this family; 

 whether connected together by a common geological 

 bond, or passing into each other by variations of com- 

 position. Four divisions will include all its varieties ; 

 the essential minerals being chiefly those which form 

 granite : and these demand a notice seldom required, 

 in this work, respecting other rocks, from the connex- 

 ion of the two leading varieties at least with its geolo- 

 gical history. 



The granitic division is distinguished by a large 

 grain and imperfectly foliated texture, and by partial 

 transitions into granite, from which, in detached spe- 

 cimens, it is often undistinguishable. It assumes this 

 structure most perfectly when in contact with that rock ; 

 and this variety is therefore most common in the lowest 

 positions. The schistose division alternates with and 

 graduates into micaceous schist and into quartz rock; 

 often in such a manner, that in describing a given dis- 

 trict, we are at a loss what term to apply to the whole. 

 It is small grained, with a granular texture when con- 

 nected with quartz rock, and with a laminar one when 

 with micaceous schist, and as the felspar is often al- 



