300 SKY. GEOLOGY. GNEISS. 



the compound ; and in a third case, quartz and felspar 

 alone are united, sometimes in an indiscriminate mixture, 

 at others by a dispersion of distinct grains of red felspar 

 through a highly crystalline translucent quartz. This rock 

 has been already mentioned as occurring in Tirey. 



I think it unnecessary to enumerate any more varieties, 

 and would willingly have avoided the detail of these, had 

 it not been requisite to prevent misapprehension from the 

 use of the general term gneiss. Without such precautions 

 mineralogists can scarcely hope to understand each other; 

 they are, like definitions, calculated to prevent mistakes. 



In recurring to the changes in the composition of the 

 gneiss between Loch Oransa and the point of Sleat, it 

 must be remarked that this line is that of the direction 

 of the beds; and that the change takes place therefore 

 not by alternation or succession of beds, but in the pro- 

 longed course of the same set. Whatever difficulty may 

 attend the explanation of this circumstance, it is not con- 

 fined to this rock ; nor is this, even here, a solitary case, 

 since it will be seen, in describing the strata which follow, 

 that they also are subject to the same irregularity ; and 

 in a degree fully as great, or even greater, since more 

 at variance with the usual appearance of rocks that are 

 in general very constant in their characters. 

 . I must here drop the subject of the gneiss unconcluded, 

 since that which remains is so intimately linked with 

 the history of the next series of rocks, on account of the 

 transition between the two, that it would not now be intel- 

 ligible. I shall therefore proceed to consider this series 

 in an order as regular as its ambiguous character permits. 



In adopting a general term, something must here, as 

 usual, be sacrificed to the convenience of using one only ; 

 I have therefore chosen the name of that which appears 

 the most characteristic, if it is not the predominant sub- 

 stance, to wit, red sandstone, as it would be otherwise impos- 

 sible to render the geological description intelligible. The 

 latitude of this term is undoubtedly great, since various 



