Ch. XII.] NATURE OF STRATIFICATION. 24-1 



two parts, the one a single horizontal lamina, and the other 

 a series of inclined ones, or there is a regular alternation of a 

 set of inclined laminae with one horizontal one. This appear- 

 ance is neither 'rare nor dubious ; it is extremely well marked, 

 and predominates throughout the whole range : and it is 

 precisely similar to the disposition of the laminae of the 

 argillaceous schist in Isla. It hence follows that, if the 

 fissile property of clay- slate is the result of some internal 

 arrangement analogous to crystallisation, we are equally 

 entitled to attribute the structure of the sandstone to the 

 same cause." * 



We are not, therefore, singular in refusing to allow this to 

 be a mark of stratification, produced by a peculiar mode of 

 deposition : indeed, if admitted, by the same rule, unstratified, 

 volcanic, and trap-rocks become stratified, and also plutonic 

 granite ; since one of the beds of Arran, being divisible into 

 laminae, presents a similar appearance of unconformity of 

 structure, when compared with the adjacent beds of this 

 rock. 



We now approach the concluding argument. " Another 

 striking point of analogy between the stratification of the 

 crystalline formations, and that of the secondary and tertiary 

 periods, is the alternation in each of beds varying greatly in 

 composition, colour, and thickness." " Reasoning, then," 

 says Lyell, " from the principle that like effects have like 

 causes, we attribute the stratification of gneiss, mica-schist, 

 and other associated rocks, to sedimentary deposition from a 

 fluid." 



Surely no one who has carefully studied the primary rocks 

 can have failed to observe that, though the rocks forming the 

 schistose groups appear to alternate on a certain line, yet 

 their beds do not exhibit that persistency of form so charac- 

 teristic of sedimentary deposits; for if he should proceed 

 either to the right or to the left of this line, with the view of 



* Geol. Trans., vol. iii. p, 47. 

 B 



