petrographical evidence in south india. 239 



Microscopical Characters. 

 As with the chemical composition, so also with the microscopic 

 structures : certain general characters distinguish rocks of igneous 

 from rocks of sedimentary origin, and they are often only partially 

 masked by the results of metamorphism. Though the deformation of 

 igneous masses may result in the complete destruction of the micro- 

 scopic characters, traces of the old structures can often be detected 

 with the microscope. Such characteristic igneous structures as basic 

 schlieren, contemporaneous veins and primary breccia have already 

 been described in the charnockite series, and it is only locally that they 

 are deformed sufficiently to permit the destruction of these features. 



In extreme cases of metamorphism, the development of new 

 minerals by molecular re-arrangement of the chemical compounds may 

 result in the formation of a rock mineralogically distinct from the 

 original material, but still the alteration products of an igneous would 

 present certain contrasts to those of a sedimentary rock. Hornblende 

 almandine garnet, epidote, sphene, and muscovite are examples of 

 common secondary minerals manufactured by alteration of igneous 

 rocks, whilst the metamorphism of calcareous sediments produces 

 wollastonite and colophonite, and of clays or other aluminous 

 materials, kyanite, sillimanite, hercynite, corundum and rutile are 

 characteristic products. 



A review of the charnockite series with these points in mind dis- 

 covers' no features which indicate a sedimentary origin, but neverthe- 

 less brings out the fact that they differ from normal igneous rocks in 

 two important structural details — (1) igneous masses of any great 

 size generally show at some point or other a porphyritic phase ; and 

 (2) generally exhibit a more or less definite order of succession 

 amongst the constituent minerals. These two points are inter- 

 dependent and may be considered together. 



A well defined porphyritic structure has been observed in the 

 Even-grained structure charnockite series in one case only, the por- 



and oscillations in order ... . 



of crystallization. phyntic crystals being orthoclase set in a matrix 



( 121 ) 



