150 GNEISS. 



caceous schist and with quartz rock, are equally abun- 

 dant and obvious in Scotland, especially where it is 

 about to be finally succeeded by either of those; and 

 they are the source of great difficulty in attempting to 

 note the respective boundaries in a geological map. 

 It is not that a few beds alone occur before the final 

 termination of the gneiss; since the alternations ex- 

 tend over considerable tracts of country, in Sutherland, 

 and in Perthshire. In these cases, it is the schistose 

 variety which is present; while there is not only an 

 alternation, but a transition from the one rock to 

 the other two, as is easily understood from the simi- 

 larity of their mineral composition. Nor is there any 

 reason in the general theory of the composition of rocks, 

 as formerly laid down, why these alternations should 

 not happen. It is thus easy to see how geologists may 

 give different reports on a country of this nature, even 

 when unbiassed by hypotheses. They who may see 

 chiefly the beds of gneiss, will consider the other rocks 

 as subordinate, and describe the country as a tract of 

 that substance; while the reverse will happen where 

 the principal attention has been given to the associated 

 rocks. Yet the one has no further title to be considered 

 as the principal formation than the others ; and it will 

 be always the safest practice to describe the simple facts, 

 rather than to incur the hazard of errors by general 

 statements. Under great minuteness and frequency 

 of the alternations, it is in vain to be anxious about 

 distinctions where Nature has made none; and we 

 must be contented to admit on the large scale, what we so 

 often find on the small one. That which Bought to be, is 

 the eternal obstacle to the discovery of that which is. 

 In numerous parts of Scotland, the gneiss is imme- 

 diately succeeded by argillaceous schist, exhibiting the 

 usual intermixture of fine and coarse strata. In lona^ 

 the schist is interposed between the granite and the 



