SUMMARY. 245 



in the change from an ophitic to a granulitic structure can 

 b^ traced very clearly in many of our ancient diabase 

 dykes : it is a secondary change in which the dirty lath- 

 shaped felspar crystals become transformed into water- 

 clear granules, and to such a process the granulitic struc- 

 ture of many old igneous rocks may be due. In some 

 instances, however, granulation may be the result of move- 

 ment towards the close of the consolidation processes. 

 Although the granulitic (panidiomorphic) structure is so 

 very general, Mr. Middlemiss has called my attention to 

 a well marked porphyritic phase in the charnockite series 

 near Chennimalai, Coimbatore District. 1 

 (2) To account for the frequent presence of garnets, sufficient 

 evidence has been obtained to indicate their formation 

 at the expense of the pyroxenic constituents. Assuming 

 that pyroxene is stable at.high temperatures and hornblende 

 the stable form of the same compound at lower tempera- 

 tures, Adams concluded that the persistence of pyroxene 

 in the highly crushed portions of the Canadian anorthosites 

 indicates the action of dynamo-metamorphism at high, 

 temperatures. Whilst adopting such an explanation for 

 the preservation of pyroxene in the charnockite series, I 

 would suggest that by continued exposure to some inter- 

 mediate temperature, a change occurs in the complex 

 ferromagnesian silicate, with molecular segregation into 

 a more basic compound, which crystallizes as garnet, and a 

 more acid compound, which simultaneously forms quartz 

 or an acid plagioclase felspar, and which forms isolated 

 inclusions in, or a graphic intergrowth with, the garnet. 

 (3) The linear disposition of the constituents, as well as the 

 alternation of mineralogically dissimilar bands, have been 

 so frequently observed in unequivocal intrusive rocks that 



1 No. 13-177. The rock has a specific gravity ©f 274, and contains the ordinary consti- 

 tuents of charnockite with porphyritic orthoclase crystals measuring i to f inch across. 



( 127 ) 



