464 Cleavage and Foliation, paet n. 



where veins of quartz abound. 1 On the flanks of the 

 mountains both in Tierra del Fuego and in other 

 countries, I have observed that the cleavage-planes fre- 

 quently dip at a high angle inwards ; and this was long 

 ago observed by Yon Buch to be the case in Norway : 

 this fact is perhaps analogous to the folded, fan-like or 

 radiating structure in the metaniorphic schists of the 

 Alps, 2 in which the folia in the central crests are vertical 

 and on the two flanks inclined inwards. Where masses 

 of fissile and foliated rocks alternate together, the cleav- 

 age and foliation, in all cases which I have seen, are 

 parallel. Where in one district the rocks are fissile, 

 and in another adjoining district they are foliated, the 

 planes of cleavage and foliation are likewise generally 

 parallel : this is the case with the feldspathic homo- 

 geneous slates in the southern part of the Chonos group, 

 compared with the fine foliated mica-schists of the 

 northern part ; so again the clay-slate of the whole 

 eastern side of Tierra del Fuego cleaves in exactly the 

 same line with the foliated gneiss and mica-slate of the 

 western coast; other analogous instances might have 

 been adduced. 3 



With respect to the origin of the folia of quartz, 

 mica, feldspar, and other minerals composing the meta- 

 niorphic schists, Professor Sedgwick, Mr. Lyell and 

 most authors believe, that the constituent parts of each 

 layer were separately deposited as sediment, and then 



1 I have seen in Brazil and Chile concretions thus enfolded by 

 foliated gneiss ; and Macculloch (' Highlands,' vol. i. p. 64) has 

 described a similar case. For analogous cases in clay-slate, see 

 Prof. Henslow's Memoir in ' Cambridge Phil. Trans.' vol. i. p. 379, 

 and Macculloch's ' Class, of Rocks,' p. 351. With respect to both 

 foliation and cleavage becoming tortuous where quartz-veins abound, 

 I have seen instances near Monte Video, at Concepcion, and in the 

 Chonos Islands. See also Mr. Greenough's ' Critical Examination,' 

 p. 78. 



2 Studer in ' Edin. New Phil. Journal,' vol. xxiii. p. 141. 



3 I have given a case in Australia. See Chapter VII. of this 

 work. 



