178 ttUARTZ ROCK. 



as transitions are also more common, and the distinc- 

 tion less easily made. In this case the modification 

 of the quartz rock is that which contains mica ; so 

 that as this increases in quantity, the common limit 

 of the two is evanescent. The geologist must here 

 he guided by the predominant characters of the whole 

 mass, not by that of a specimen ; nor must he be sur- 

 prised if he cannot always find fixed distinctions be- 

 tween rocks so radically the same in composition. 

 These alternations are sometimes therefore so gradual 

 as to present a complete series of transitions ; while, 

 in other cases, as in Jura and Scarba, they are per- 

 fectly defined, even on a very minute scale. In such 

 cases of minute alternation, it is as common for the 

 micaceous schist to be subordinate in quantity to the 

 quartz rock, as the reverse. 



Alternations of quartz rock with argillaceous schist 

 have occurred to me only in Glen Tilt, and in the 

 chain of islands already mentioned ; yet always on a 

 small scale, and in beds of inconsiderable thickness, 

 sometimes extremely thin, and beautifully regular. In 

 the islands of the chain of Jura, there is a triple or 

 quadruple series, consisting of quartz rock, with mi- 

 caceous and with fine and coarse argillaceous schists, 

 in endless alternations ; so that every one of those 

 rocks is proved to be alternately more antient and 

 more recent than the others ; still further showing, 

 could that be necessary, the fallacy of 'those hypo- 

 theses which lay down a fixed order of succession 

 among the primary strata. Where it alternates with 

 the coarse argillaceous schist, or gray wacke, its beds 

 pass into that rock through the conglomerate struc- 

 ture already described. Its analogy to the secondary 

 sandstones, here, becomes apparent ; the same mixed 

 composition pervading both, as in the occasional con- 



