CORRELATION WITH FOREIGN ROCKS. 20$ 



from those of Saxony chiefly in being a little coarser in grain, and 

 in possessing, as a general rule, a more or less indistinct schistose 

 structure, whilst garnet is less abundant. In these points the 

 Canadian pyroxene-granulites agree with many occurrences in South 

 India. In one case Adams has figured a dyke-like arm protruding 

 into the leaf-gneisses from the pyroxene-granulites, but the arm has 

 been folded up with the gneisses through which it cuts. 



Of the numerous other occurrences of pyroxene-granulite, now 

 known in various parts of the world, many have been regarded 

 as probably igneous in origin, purely on account of their minera- 

 logical resemblance to gabbros, norites and other known eruptives ; 

 but no direct field evidence has been discovered to prove the true 

 nature of these peculiar rocks. 



Under the name " pyroxene-gneiss " Lacroix has described a 

 number of specimens which were collected in 

 *MtL*72^ the Madras Presidency as long ago as 1819 by 

 Leschenault de la Tour. Some of these are evi- 

 dently identical with members of the charnockite series. On peno- 

 logical grounds alone they are correlated by Lacroix with the 

 gneisses distinguished by the letter £* in the geological map of 

 France, and are regarded as older than the associated hornblendic, 

 chloritic and mica schists. At the same time the pyroxenic rocks 

 are looked upon as members of the upper part of the gneissose 

 series. The facts that the specimens described by Lacroix only 

 imperfectly represent the large assemblage of pyroxenic rocks which 

 field observations, as well as microscopic evidence, show to be 

 genetic relatives in the Madras Presidency, and that their localities 

 have been imperfectly recorded without field notes, detract seriously 

 from the geological value of a memoir, which, as a contribution to 

 our knowledge of Madras mineralogy, is highly appreciated by the 

 Geological Survey of India. As nearly as the few data would 

 permit I have attempted to identify the localities of the rocks de- 

 scribed by Lacroix, and my researches in the field confirm his con- 

 clusion that the pyroxene-granulites are amongst the youngest of the 

 foliated crystalline rocks in South India; their peculiar position 



( 87 ) 



