250 SYNOPSIS OF GNEISS. 



only or with hornblende only, in the same situations ; but 

 it is scarcely necessary to specify these varieties as they 

 appear to be very limited in extent. 



With respect to the first division I may add, that the 

 several varieties enumerated under it are occasionally 

 found passing into each other, in all situations where 

 gneiss occurs ; notwithstanding the leading prevalence of 

 one or other of the subdivisions. 



SECOND DIVISION. 



Of irregular composition, containing compacted and 

 compact felspar. 



FIRST SUBDIVISION. 



Containing compacted felspar united to the ordinary 

 ingredients of gneiss; the structure granitic or else par- 

 tially schistose. 



A. Compacted granular felspar with or without quartz, and 



interlaminated with hornblende. 



B. The same with mica instead of hornblende. 



C. The same with both mica and hornblende. 



The varieties here included are far less common than 

 the granitic or schistose gneiss, but they occur never- 

 theless in considerable quantity in Sutherland and in Ross- 

 shire, very often in company with those of the following 

 subdivision. The essential distinction between them and 

 the ordinary varieties of gneiss consists in the condition 

 of the felspar, which is disposed in small crystalline par- 

 ticles, often of a glassy lustre, compacted into a dense 

 and somewhat granular aggregate. Hence it is distin- 

 guished by the name of compacted felspar, a term which 

 expresses this peculiarity without confounding it with 

 ordinary compact felspar. These rocks all present a very 

 decided character by which they are easily distinguished 

 from the more common varieties of gneiss. 



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