THE METAMORPHIC ROCKS OF ADEL1E LAND.-STILLWELL. 193 



2. THE CHARNOCKITE SERIES. 



The charnockites are a group of crystalline rocks which appear among the Archtean 

 gneisses in Southern India. The distinguishing features of the unaltered members 

 of the series (p. 125) are the constant even-grained granulitic structure and the constant 

 presence of the mineral hypersthene, while garnet uniformly appears in the gneissose 

 forms, just as in the Antarctic rocks. The chief types range from acid charnockite 

 and leptynite (granulite) to basic norites and ultrabasic pyroxenites and hornblendites. 

 They are a series which possesses resemblances to the Saxon pyroxene granulites and 

 the French pyroxene gneisses, and to other ancient pyroxenic eruptives ; but they are 

 determined from evidence within the series itself to be of igneous origin. At times they 

 are acknowledged to have suffered some alteration, but the igneous character is held 

 to be dominant. 



The pyroxene granulites and the pyroxene gneisses are looked upon as crystalline 

 schists, and hence it must be considered possible, apart from Antarctic evidence, that 

 the charnockites are similarly so. Besides, we are told (p. 195) that the charnockites 

 are quite old enough to be affected in the same way as the Dharwar system of crystalline 

 schists. 



The charnockite series groups together acid and basic rocks in a way that is known 

 to be (p. 154) contrary to the usual practice of petrographical (igneous rock) classification, 

 and Holland (pp. 131, 210) is quite aware that metamorphism tends to reduce points 

 of difference between rocks of diverse origin and to produce similarity. Holland is also 

 aware that the granulitic structure (p. 154) with general absence of idiomorphism favours 

 a metamorphic origin, but he believes that a granulitic structure may result from dis- 

 turbance of the magma during the process of consolidation. This belief rests on the 

 observation that dykes of pyroxenite cut the norites, and the igneous origin of the 

 pyroxenite cannot, therefore, be doubted. But this observation does not preclude 

 the possibility that the pyroxenite dyke and the norite have suffered similar meta- 

 morphic conditions during which the dyke characters have been preserved in the same 

 manner as at Cape Gray. Such grouping, then, of the charnockite series indicates 

 the outstanding nature of the genetic relationship between the various members. 



Now we find (p. 125) that nearly all varieties possess a linear arrangement of the 

 constituent minerals, i.e., a foliation. In the case of rocks of St. Thomas Mount and 

 Pallavaram the direction is constant between N.N.E. and S.S.W. Holland insists 

 that this foliation is not a metamorphic feature, because it may occur (p. 125) in rocks 

 with a complete absence of all signs of crushing, and because (p. 137) the most delicate 

 interlocking structures may be preserved, and all signs of dynamo metamorphism are 

 wanting. Hence Holland concludes that this disposition of the minerals occurred 

 before consolidation. It seems to us that the degree of dynamo metamorphism is here 

 determined wholly by the amount of induced mechanical structures, e.g., granulitisation, 

 mylonisation, etc., and that this is an instance that would justify Wienschenk's criticism 



Serin A, VoL m., Part 1 N 



