ch.\p. xm. Granitic Rocks of . 427 



rock, is, as far as I know, a unique case : and I will 

 not attempt any explanation of its origin. 



No. 38. 

 Fragment of Gneiss embedded in another variety of the same rock. 





The numerous travellers 1 in this country, have all 

 been greatly surprised at the depth to which the gneiss 

 and other granitic rocks, as well as the talcose slates of 

 the interior, have been decomposed. 'Near Rio, every 

 mineral except the quartz has been completely softened, 

 in some places to a depth little less than one hundred feet. 2 

 The minerals retain their positions in folia ranging in 

 the usual direction ; and fractured quartz veins may be 

 traced from the solid rock, running for some distance 

 into the softened, mottled, highly coloured, argillaceous 

 mass. It is caid that these decomposed rocks abound 



1 Spix and Martins have collected, in an Appendix to their 

 ' Travels,' the largest body of facts on this subject. See, also, some 

 remarks by M. Lund, in his communications to the Academy at 

 Copenhagen ; and others by M. Gaudichaud, in Freycinet's ' Voyage.' 



2 Dr. Benza describes granitic rock (' Madras Journal of Lit.' &c. 

 Oct. 1836, p. 246), in the Neelgherries, decomposed to a depth of 

 forty feet. 



