DESCRIPTION OF THE CHIEF TYPES. 143 



quartz to microcline and opaque iron-ores is the same in this rock as 

 in the unaltered charnockite, but instead of hypersthene we have 

 about an equal quantity of irregularly-shaped pink garnets (see Nos. 

 9/665 and 9/66S). The rocks now referred to present the characters 

 of those known to German petrographers as " normal granulite," but 

 the minerals rutile, kyanite, sillimanite, etc., so frequently found in 

 the Saxon granulites do not occur in these rocks at Pallavaram. I 

 have previously detailed the evidence which shows that garnets are 

 developed in rocks of this group at the expense of the pyroxene. 1 



A comparison of this leptynite with the fresh and unaltered 

 charnockite affords interesting examples of the difference between 

 the results of the pressure which brings about a parallel disposition 

 of the rock-constituents before complete consolidation, and that by 

 which the stable minerals, after the solidification of the rock, are 

 smashed into a mylonised mosaic, whilst the pyroxenes are converted 

 into the com monest of all the products of metamorphism, garnets. 



The different circumstances under which garnets appear in 

 the charnockite series enable us to indicate the conditions which are 

 favourable to their manufacture. In acid exposures near Pallavaram 

 we have seen that the garnets appear in a rock which only differs from 

 the charnockite in being crushed ; but in the basic members of this 

 series garnets are abundant in rocks which do not show the slightest 

 signs of crushing. Clearly then simple dynamo-metamorphism is not 

 essential for their production. This at once sug- 



u3££3b£f£L£ S ests an en 1 uir y int0 the influences of special 

 temperature conditions. We know that by the 

 fusion of garnet we obtain pyroxene amongst the products of the 

 devitrification of the melt. It is also known, from the experiments of 

 Fouque and Michel-Levy, that whilst pyroxene is stable at high 

 temperatures, hornblende is the stable form of the same compound at 

 low temperatures. If then our pyroxenic rocks were subjected to 

 dynamo-metamorphism at low temperatures, the pyroxene would be 

 amphibolized andhornblendic rocks would result. Probably, therefore, 



1 "On the origin and growth of garnets and of their micropegmatitic intergrowths in 

 pyroxenic rocks." Rec. Gecl. Surv. Ind-, Vol. XXIX (1896), p. 20. 



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