1 66 



HOLLAND: CHARNOCKu'E SERIES. 



The non-felspathic forms of the pyroxenic rocks may be divided 

 into the following leading types : — 

 (a) Types rich in hypersthene. 

 (£) Types rich in hornblende passing into amphibolites. 



(a) Types rich in hypersthene. 



The purer forms of pyroxenite forming dykes in the norites 

 west of Pallavaram are perfectly granulitic in structure and are com- 

 posed of hypersthene, brown hornblende and augite, with small quan- 

 tities of olivine, green and black spinelloids and occasionally apatite. In 

 rocks where the microscopic structure is so perfectly granulitic as in 

 these pyroxenites it is impossible to determine the relative ages of 

 the constituent minerals with certainty. It is an interesting circum- 

 stance to find that the granulitic structure of the norites described 

 above is so perfectly displayed also by their ultra-basic relatives 

 whose intrusive characters can hardly be doubted. 



A chemical analysis of No. 9,394, by Dr. T. L. Walker, gave the 

 following results : — 



Si0 2 



• • • • 



46-86 



A1 2 3 



• • • • 



9/80 



Fe 2 3 and FeO . 



16-35 



CaO . 



• • • • 



9-57 



MgO. 



• • « • 



18*08 



Alkalies 



• # • • 



. . traces 



Ignition 



• • • • 



0*67 





101-33 



Sp. Gr. = 3-333 

 Hypersthene is by far the most abundant constituent of the 

 Pallavaram pyroxenites (Nos. 8*756, 9*394, 9* 6 7 2 )- lt is a ni g nl y 

 pleochroic variety, probably approaching in composition the varieties 

 of rhombic enstatite for which Judd has proposed to revive Vom 

 Rath's term amblystegite. The crystals are very frequently schil- 

 lerized, with the development of brown-red plates similar to those 

 of the well-known hypersthene of St. Paul. 



( 48 ) 



