cxxii Journal of the Asiatic Society of Bengal. N..3, XII , 



axis in the hypersthene, so that there may be a confusion between augite 

 and hypersthene without a complete rotation of the nicols. Hypersthene 

 may also be mistaken for garnet if the nicols are not rotated. 



The question of the extinction of rhombic pyroxenes has puzzled many 

 petrographers. Sir Thomas Holland, in his memoir on the charnockite 

 series, writes that although he has examined hundreds of cases from the 

 Salem district and from all parts of the Madras Presidency, he has never 

 found a pyroxene in these rocks giving the pleochroism of hypersthene 

 without at the same time, when definite cleavage lines are exhibited, show- 

 ing a straight extinction. He has very carefully searched every specimen 

 in the extensive collection made by his colleagues and himself, and has to 

 confess his inability to discover a single instance of such a pleochroic 

 monoclinic pyroxene. Dr. Walker in his memoir on the Geology of Kala 

 handi State speaks of a mineral more or less resembling the rhombic 

 pyroxenes in regard to inclusions, pleochroism and colour, but seldom 

 extinguishing straight along the chief lines of cleavage, to which he as- 

 cribes the name clino-hypersthene. In several cases the angle of extinc- 

 tion of the mineral showing the same pleochroism as hypersthene has been 

 found to be 45°, and this is perhaps the pleochroic pyroxene described by 

 Lacroix in the pyroxene gneisses of Salem. There seems to be some eon- 

 fusion about the extinction of 45° on the clino-pinacoid. The clino-pina- 

 eoid (010) of Lacroix is perhaps the cleavage trace m (110) and if it is so, 

 the monoclinic pyroxene becomes the rhombic pyroxene, — hypersthene. 

 There is also a paper by Messrs. Allen, Wright, and Clement, entitled 

 '^Minerals of the Composition MgSiO^: a case of Tetramorphism " in 

 Vol. XXII, American Journal of Science, 1906, where one of the four 

 forms of MgSiO;* is a monoclinic pyroxene which may be regarded as a 

 clino-enstatite, but there is no information concerning clino-hvpersthene 

 which would be the monoclinic form of (Mg, Fe) SiO& It is well known 

 that rhombic minerals exhibit both straight and oblique extinctions. It 

 is therefore a dangerous assumption to consider a mineral as clino-hypers- 

 thene when the hypersthene extinguishes obliquely. But the actual exam- 

 nation of some slides suggests that there is an intermediate and tran- 

 sitional alteration product between augite and hypersthene, showing 

 oblique extinction but hardly approaching true hypersthene in all its 

 characters. 



In an acid charnockite, green pleochroic hornblende derived from a 

 pre-existing augite is observed to pass into garnet which is present in the 

 form of large blebs and stringers forming a complete network. The horn- 

 blende often occurs right inside garnet, occasionally at the margin. In a 

 specimen of norite, green augite is observed to pass into pink hypersthene, 

 the .transitional stage being marked by a faint pink colour , occasionally 

 by a dark pleochroic band. The peculiar cleavage lines of augite can also 

 be traced in the altered form with which the augite is crystallographically 

 continuous. The pleochroic mineral does not invariably show straight 

 extinction, but often reaches as much as 26°. In another section, hypers- 

 thene is found in sparing quantity, but usually shows a pronounced pleo- 

 chroism. It i3 altered from augite, where both the augite and hypers- 

 thene extinguish at an angle of 40°. In another specimen garnet appa- 

 rently sends out vermicular blebs of augite, often of hypersthene, into 

 the felspar which is also secondary, and where there is magnetite these 

 blebs stretch out from garnet to magnetite. An extreme type of charnock- 

 ite composed entirely of quartz and augite with accessory magnetite, 

 graduates into a garnetuerous quartz rock with the production of hypers- 

 thene, felspar, and pyrite. There are transitional stages between the 

 garnet and the light-green granular stuff, the disintegration product of 

 augite. A single idiomorphic augite has been observed to alter into 

 numerous hypersthene individuals, all differently orientated. Sir Thomas 

 Holland mentions the occurrence of frequent intergrowths of rhombic (ens- 

 tatite) and monoclinic pyroxenes in some exceptional hemicrystalline 

 varieties of augite-norite. These intergrowths perhaps represent the 



