Dr. M'^Culloch's Supplementary Remarks on Quartz Rock. 69 



supposing the original quartz rock to have contained felspar in different pro- 

 portions; and the peroxidation of the iron^ equally accounts for the red colour 

 of the altered rocks. The fracture and displacement are in all respects 

 analogous to those which occur among strata in the vicinity of trap ; and the 

 whole presents an additional mass of evidence respecting the analogous powers 

 of trap and granite in altering the characters and conditions of the strata with 

 which they interfere. If the action of heat be admitted in the one case, it is 

 not easy to see how it is to be refused in the other, when the effects in both so 

 exactly correspond. 



The last association of quartz rock which has been discovered by the inves- 

 tigations of 1819, is of a more intricate nature ; and with a sketch of that I 

 shall finally terminate these supplements. The Transactions of the Geolo- 

 gical Society having formed the repository of these progressive remarks on a 

 rock before obscure, it is unnecessary to apologize for the wish to place the 

 whole in the same work ; although a full view of this part of the subject is 

 given in the author's account of the Western Islands of Scotland. 



The peculiar series here to be described consists of a repeated alternation 

 of this rock with micaceous schist, chlorite schist, and hornblende schist ; the 

 two latter presenting many remarkable varieties of composition and texture. 

 This series extends over a considerable tract of country, in very regular stra- 

 tification. 



The beds of these different rocks rarely exceed a very few yards, and are 

 often but a few inches in thickness. In general the quartz rock forms a bed of 

 the largest of these dimensions, and is accompanied by a small one of micaceous 

 schist. The other substances follow in an order more or less regular ; and 

 thus the series is repeated, with various inclinations, for a space of not less 

 than twenty miles across the whole. The linear direction is remarkably 

 exact, and the beds exhibit no marks of disturbance. 



In these beds few examples occur, as far as I examined it, of that variety of 

 quartz rock which contains felspar; and those which are found differ much in 

 aspect from what occurs in association with gneiss, in which that mineral 

 forms so conspicuous an ingredient. The predominant beds are simple, com- 

 pact, granular or splintery, but occasionally also micaceous. 



In concluding this final supplement it will be useful to enumerate the lead- 

 ing varieties of quartz rock, as far as respects its mineral characters ; but the 

 fuller details of these must unavoidably be reserved for a more appropriate 

 place. 



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