chap. xiii. Cleavage and Foliation. 465 



metamorphosed. This view, in the majority of cases, I 

 believe to be quite untenable. In those not uncommon 

 instances, where a mass of clay-slate, in approaching 

 granite, gradually passes into gneiss, 1 we clearly see 

 that folia of distinct minerals can originate through the 

 metamorphosis of a homogeneous fissile rock. The de- 

 position, it may be remarked, of numberless alternations 

 of pure quartz, and of the elements of mica or feldspar, 

 does not appear a probable event. 2 In those districts 

 in which the metamorphic schists are foliated in planes 

 parallel to the cleavage of the rocks in an adjoining 

 district; are we to believe that the folia are due to 

 sedimentary layers, whilst the cleavage-laminae, though 

 parallel, have no relation whatever to such planes of 

 deposition ? On this view, how can we reconcile the 

 vastness of the areas over which the strike of the folia- 

 tion is uniform, with what we see in disturbed districts 

 composed of true strata : and especially, how can we 

 understand the high and even vertical dip throughout 

 many wide districts, which are not mountainous, and 

 throughout some, as in western Banda Oriental, which 

 are not even hilly ? Are we to admit that in the 

 northern part of the Chonos Archipelago, mica-slate was 

 first accumulated in parallel horizontal folia to a thick- 

 ness of about four geographical miles, and then upturned 

 at an angle of forty degrees ; whilst, in the southern 

 part of this same archipelago, the cleavage-laminas 

 of closely allied rocks, which none would imagine had 

 ever been horizontal, dip at nearly the same angle, to 

 nearly the same point ? 



Seeing, then, that foliated schists indisputably are 



1 I have described (in Chapter VII. of this work) a good instance 

 of such a passage at the Cape of Good Hope. 



2 See some excellent remarks on this subject, in D'Aubuisson's 

 ' Traite de Geog.' torn. i. p. 297. Also, some remarks by Mr. Dana 

 in Silliman's ' American Journ.' vol. xlv. p. 108. 



