240 HOLLAND: CHARNOCKITE SERIES. 



resembling the ordinary charnockites (No. 13*177). Lacroix l has 

 referred to the oscillations in the order of succession which charac- 

 terise these pyroxenic gneisses and which distinguish them from 

 normal igneous rocks. Exactly why there should be this difference 

 between simple eruptives and the old crystalline rocks has not been 

 fully explained ; but in this case, as in the case of the elaeolite-syenite 

 of Sivamalai, 2 the general absence of idiomorphism and the apparent 

 contradictions in the order of crystallization may in some cases be 

 explained by movement of the magma during the process of con- 

 solidation, just as, according to Professor Judd, the ophitic frame- 

 works of augite around plagiociase break up to form a granulitic 

 aggregate when dolerites and basalts are disturbed by movement 

 during the process of crystallization. 3 In many of these very 

 ancient rocks a panidiomorphic structure has been produced by 

 recrystallization of the minerals. Stages of the process are often 

 observed in the old dyke-rocks of South India. 



Becker * has suggested that the production of the porphyritic 

 structure of some lavas and dyke-rocks is favoured by the freedom of 

 molecular translation arising from a high degree of fluidity, whilst the 

 even-grained texture of massive rocks is due to consolidation of a 

 less perfectly molten magma in which molecular movement is com- 

 paratively restricted. For this reason Becker thinks that the por- 

 phyritic crystals are formed when the magma is very mobile, whilst 

 the granular groundmass of the same rocks is formed when, by 

 reduction of temperature, the viscosity of the magma becomes in- 

 creased. The extension of this interesting speculation to the char- 

 nockite series would form a partial explanation both of the absence 

 of porphyritic structure and of the limited degree of differentiation 

 which has taken place in the great masses. 



But the banding and foliation, without crush structures, amongst 



1 Rec. Geol. Surv. Ind., Vol. XXIV, p. 162. 



2 To be described in Mem. Geol. Surv. Ind., Vol. XXX, part 3. 



8 Quart. Journ Geol. Soc. t Vol. XL1I (1886), pp. 68, 76, and plate V. 

 4 Amer. Journ. Scu, Vol. XXXIII (1887), p. 50. 



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