CLASSIFICATION OF THE ROCKS. X35 



quantities and under high pressure, as is shown to be the case by 

 the quartz,, it would naturally attack the unstable silicate of magnesia 

 (olivine) forming carbonate of magnesia and free silica, which is found 

 abundantly in the magnesite and its associates. Whether the 

 quartz is derived from the same magma as the peridotite, or whether 

 it is a separate subsequent eruption, the argument still holds. This 

 account of the formation of the magnesite disagrees with that put 

 forward by Messrs. King and Foote only in recognising the fact that 

 the rocks which have been altered were originally peridotites 

 and not metamorphic rocks. After giving an account of a large 

 number of such magnesite areas included within sheet 79 of the 

 Indian Atlas, Messrs. King and Foote say: " These phenomena of 

 the re-metamorphism of metamorphic rocks are of such a peculiar 

 character that there can be but small grounds for hesitation in 

 ascribing them to the action of water (probably of such elevated tem- 

 perature as to have acted in some cases in the form of steam) aided 

 by the presence of corrosive gases, especially carbonic acid gas." 1 



This .inference, based on purely theoretical considerations, is 

 abundantly confirmed by the discovery of liquid carbonic acid in the 

 associated quartz ; for in these cavities we have specimens of the 

 carbonic acid preserved in a form which shows that, at the time at 

 which the quartz crystallized, the gas was under very high pressure 

 as well as at a high temperature. 



Small quantities of serpentine have also been formed by 

 Limited degree of hy- hvd ration of portions of the olivine which 

 dration. escaped the action of the carbonic acid ; but 



compared with the magnesite, the serpentine is extremely small in 

 quantity. It has been abundantly shown by the freshness of olivine, 

 elaeolite and other delicate minerals in very old rocks, that decom- 

 position by subaerial agencies has been very superficial and limited 

 in South India ; 2 and the recognition of the evidences which show 



1 Op. cit., p. 325. 



2 See Rec, Ceol. Surv., Vol. XXX. p. 40; Mem., G. S. /., Vol. XXVIII, pt. 7. See also 

 Col. Mag., decade IV, Vol. VI, 1899, p. 540. 



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