GNEISS. 253 



The varieties comprehended under the term 

 gneiss are so considerable, that no general de- 

 scription of the structure of the stone can be 

 given : it is necessary to describe separately that 

 of each variety. The three marked varieties of 

 structure may be comprised under the granitic, 

 the schistose, and the laminar. 



The granitic variety is distinguishable by its 

 general resemblance to granite, which it also 

 emulates in the infinite variety, intermixture, 

 magnitude, and proportions of the several in- 

 gredients. As already remarked, it frequently 

 passes into granite by an undefinable transition : 

 and, both this transition, and the resemblance to 

 the granitic character, occur chiefly in those 

 cases where the beds of gneiss are in the vicinity 

 of granite. At the point of junction, the two 

 rocks are sometimes undistinguishable : and a 

 similar gradation often exists in those parts 

 which are traversed by granite veins. The dis- 

 tinction, as already mentioned in treating of 

 granite, consists in the general parallelism of 

 the mica, or of the hornblende ; or else of some 

 other ingredients ; from which cause the rock 



