PETROGRAPHICAL EVIDENCE IN SOUTH INDIA. 211 



of the members of the great mass of our crystalline schists may never 

 have their original nature satisfactorily settled. With the many 

 theories which have been propounded to account for all the results 

 of metamorphism, this paper does not profess to deal. The writer 

 wishes merely to point out the evidences upon which he has based 

 his conclusions as to the igneous origin of the charnockite series, 

 and to indicate briefly how far the observations made are reliable 

 data. There seems to be no reason why the knowledge we have 

 acquired as to the properties which distinguish known igneous from 

 unequivocal sedimentary rocks should net be applied to the old 

 Archaean crystallines. That these very ancient rocks present pecu- 

 liarities not found in any younger rnetamorphic rock may be true, 

 and, on account of their great age, should be expected. That all 

 their original structures have been changed beyond possible recogni- 

 tion may also be true locally ; but it is hardly likely (and no approach 

 to proof has been offered) that this destruction of original characters 

 is universal. On the contrary, it is more likely that the rocks we call 

 Archaean have undergone very different degrees of alteration in differ- 

 ent areas(some of them thereby retaining relics of original characters), 

 and as long as this likelihood remains undisputed, it will be more 

 scientific to assume that the law of uniformity holds, and that we may 

 profitably apply the experience gained from younger rocks towards 

 the elucidation of the phenomena presented by the old crystalline 

 gneisses and schists. At any rate, such a proceeding has greater 

 claims upon the student of ancient crystalline regions than any 

 single untried sweeping generalization based on a purely specu- 

 lative assumption as to the origin of these rocks. A recognition 

 of the fact that the Archaean rocks have undergone profound 

 alterations is not necessarily inconsistent with an objection to en- 

 veloping such a large fraction of our exposures in an impenetrable 

 mystery beyond the range of our present methods of petrographical 

 research. 



The determination of the origin of any particular formation 

 amongst the crystalline schists depends upon (i) the physical form 



( 93 ) 



