326 Washington — Charnockite Series of Igneous Rocks. 



The norm of A is given below. Those of the other two are 

 not given, because of the non-separation of the iron oxides and 

 the uncertainty as to the alkalies. The norms of both show an 

 excess of A1 2 3 , in B amounting to 5*92 per cent, which is 

 incompatible with their modes. 



Q 41-22 



Or 24-46 



Ab 24-10 



An •_ 5-00 



Hy 3-34 



Mt 162 



II 0-61 



This places the rock in tehamose, with the symbol I.3V2.3. 

 B would also fall in 1.3.2.3, and C in 1.3.1.2, but for these 

 analyses these symbols are of little significance. 



Hyper sthene Quartz diorite. {"Intermediate Charnockite".) 



Intermediate between the most and the least silicic members 

 of the series are many rocks which show, according to Holland, 

 a considerable diversity in characters. These do not occur at 

 St. Thomas' Mount or Pallavaram, but are widely distributed 

 over the Madras Presidency and to the west. The only repre- 

 sentative of these which I have is a specimen (11-915) from 

 Arthur's Seat, Yercaud, in the Shevaroy Hills, Madras. 



In the hand specimen this much resembles the typical char- 

 nockite, being dense and of the same brownish gray color. 

 The grain, however, is distinctly coarser, and the large speci- 

 men shows quite distinct, though irregular, variations in this 

 character. Small black grains of pyroxene are scattered 

 through a granular mass of quartz and feldspar, though it is 

 almost impossible to distinguish these with the lens. The 

 specific gravity is about 2-77. 



Microscopically, also, its characters are much like those of 

 the charnockite. The texture is granitic. The quartz shows 

 no hair-like inclusions and occasionally shows undulatory 

 extinction. The feldspar is, apparently, largely alkalic, some 

 of .it showing the microcline grating, while the greater part 

 shows no twinning lamellae. Some of this latter, as pointed 

 out by Holland, contains small colorless, elongated or fusiform 

 inclusions', with slightly higher birefringence and refraction 

 than the feldspar, arranged in parallel position. They are 

 apparently composed of quartz. Soda-lime feldspar is much 

 less abundant than the alkalic, to judge by the small number 

 of individuals with albite twinning lamellae, the extinctions 

 corresponding to the composition Ab 3 An,. Some of these also 



