20S HOLLAND : CHARNOCKITE SERIES. 



peculiarities of composition and mineral associations which they 

 display. 



Although there is such a complete correspondence in the mineral 

 lists it is generally not difficult to distinguish a microscopic sec- 

 tion of a typical basic pyroxene-granulite from one of these eruptive 

 norites. The most constant of these differences is displayed by 

 the plagioclase-felspars : in the distinctly eruptive norites the twin- 

 bands are sharp and clearly defined, whilst in the pyroxene- 

 granulites the twinning is less definite and the bands show a strange 

 tendency to thin out like wedges instead of traversing the whole 

 crystal. Recently, Dr. Walker and I found an instance in Coorg of a 

 mass of coarse norite, showing these well-twinned felspars, in the 

 midst of a large area of the charnockite series (pyroxene-granulites). 

 It was perfectly easy to distinguish a typical section of the norite 

 from the general style of the charnockite series, but we neverthe- 

 less found it quite impossible to discover a boundary between the 

 two formations ; for the coarse-grained, massive norite became 

 granulitic and gneissose near what should have been its boundary 

 with the charnockites. 



The differences between the typical norite of Coorg and the 

 charnockite series around are just the differ- 

 Anorthosites of Canada. ences which mark off the anorthosites of 

 Canada from the pyroxene-granulites in the 

 same area ; but in Canada the anorthosites make up the main forma- 

 tion and the pyroxene-granulites are comparatively restricted in their 

 development, whilst in Madras matters are reversed and we have 

 but this small exposure of norite to compare with great mountain 

 masses of the charnockite series. I have recently identified a series 

 of anorthosites (labradorite rocks) and norites near the south border 

 of the Raniganj coal-field. The norites are fine-grained and granuli- 

 tic, sometimes foliated and sometimes showing an occasional garnet. 

 The labradorite rocks are very variable in the size of their crystals ; 

 the ferromagnesian constituents which occur in comparatively small 

 quantities, include an occasional olivine with well-defined reaction 

 rims. 



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