324: Washington — Chamockite Series of Igneous Hocks. 



examining optically the hypersthene and feldspar respectively. 

 For discussion of the localities, geological occurrence, origin, 

 and other details, Holland's paper may be consulted. 



Chamockite {Hypersthene granite). 



This specimen (9*658)* is from the central part of Magazine 

 Hill, St. Thomas' Mount, eight miles south of Madras, the type 

 locality. 



It is very fine-grained, looking much like a dense quartzite, 

 of a dark, somewhat greenish, brownish gray, and a vitreous, 

 almost greasy, luster. None of the constituent minerals can 

 be definitely distinguished with the lens. Holland gives the 

 specific gravity as 2 - 6T. 



The microscope shows it to be composed in great part of 

 quartz and alkali feldspar, with a little oligoclase, and hyper- 

 sthene, and less biotite and magnetite. The texture is granitic, 

 the individuals being all anhedral and with consertal fabric. 



The quartz is clear and I could detect none of the hair-like 

 inclusions mentioned by Holland (p. 138), nor is the quartz 

 blue in my specimen. The alkali feldspar usually shows the 

 common microcline striations, though much is free from them. 

 The oligoclase shows very fine multiple lamellse, with extinc- 

 tions corresponding to Ab 6 An r The pyroxene forms small, 

 very irregular grains, colorless but with a trace of pleochroism, 

 which is so faint as to be discernible only after study of the 

 other rocks. f The scheme is c colorless, a and h faint pink. 

 The cleavage lines are very coarse and the extinction often 

 oblique to them, a peculiarity which will be discussed later. 

 There are a few flakes of brownish biotite (not mentioned by 

 Holland), and fewer grains of magnetite. 



The mode, determined by Rosiwal's method, is as follows : 



Quartz 40 



Microcline 48 



Oligoclase 6 



Hypersthene 3 



Biotite 1 



Magnetite _ 2 



It is clear, on comparison with the norm, that some of the 

 anorthite molecule must be present in the unstriated feldspar, 

 which is reckoned as microcline. This feature will be met 

 with in other members of the series. 



* These are the India Survey numbers on the specimens. 



f Judging from Holland's descriptions of the pleochroism and other 

 optical properties of the minerals, in all the specimens, it is probable that 

 the thin sections studied by him were considerably thicker than mine. 



