162 MICACEOUS SCHIST. 



to repeat former general doctrines to say, that they 

 have been produced by mechanical force when the 

 rock was in a soft state ; yet under conditions so 

 varied, that while, in some instances, it has undergone 

 the most intricate contortions, in others it has not 

 yielded without fracture. Whether this effect was 

 here produced by one or other of the conditions for- 

 merly examined, namely, by the effect of heat, or in 

 consequence of the presence of water at a certain 

 stage of its existence, is a question that needs not be 

 discussed again. 



To conclude the theory of this rock, it has been 

 disputed whether micaceous schist was a purely cry- 

 stallized and chemical compound, or whether it was 

 of a mechanical origin. As already remarked in the 

 case of gneiss, the disposition of the mica is not a 

 proof of the latter ; but the regular order of the beds, 

 implying a subsidence from water, indicates in this, 

 as in other rocks, an originally mechanical arrange- 

 ment ; as is completely proved by its alternating with 

 conglomerated substances, such as the coarse gray- 

 wackes and the quartz breccias which accompany it 

 in Jura and Scarba, and still more, by its containing 

 conglomerate portions, including fragments of granite, 

 limestone, quartz, and other rocks, as I originally 

 pointed out in Isla, Schihallien, and Fetlar in Shet- 

 land. But the proofs of a chemical structure com- 

 bined with a mechanical disposition, are not less per- 

 fect; while I need scarcely repeat that this occurs in 

 rocks which evince undisputed marks of mechanical 

 structure, by containing organic remains. To such 

 chemical causes must be referred those crystallized 

 substances, such as garnet, which so often form a 

 constituent part of the rock, and which have unques- 

 tionably been produced in tbe places where they now 



