DESCRIPTION OF THE CHIEF TYPES. 151 



Chemical composition. 

 A specimen from the typical exposures of the intermediate series 

 in the Shevaroy Hills has been analysed by Dr. T. L. Walker. The 

 specimen had a specific gravity of 2*772, being thus near the average 

 for the group, and the whole of the piece used for specific gravity 

 determination was crushed up for analysis and gave the following 

 results : — 



No. 11*915 from Arthur's Seat, Yercaud, Shevaroy Hills. 



SiO a 63-77 



Al 2 3 1630 



Fe a 3 7*49 



Ca O " . . . 6*33 



Mg O 2'49 



Na a O 368 



K O i'2i 



Ignition Nil 



101*27 



Sp. Gr. — 2772 



Microscopical characters. 



The minerals which enter into the composition of the intermedi- 

 ate varieties are similar in character to the same species represented 

 in the pure charnockite and pure norite. The quartz is generally 

 the blue or grey variety which is characterised by its hair-like 

 inclusions (supra, p. 138). The augite is the pale, blue-green variety 

 (infra, p. 156), the hypersthene, highly pleochroic (supra, p. 141), and 

 the hornblende is the strongly pleochroic, brown-green, basaltic 

 form described below (p. 158). Opaque iron-ores, zircon, apatite and 

 biotite occur as accessories, as in all varieties of this series. The 

 fact that the same species of minerals present similar microscopic 

 peculiarities in all members of the charnockite series is of course the 

 cause of the striking resemblance which specimens of these rocks 

 bear to one another in macroscopic characters, and which shows 

 their undoubted consanguinity. 



Probably the most characteristic t features of the intermediate 

 varieties are the microperthitic felspars and the quartz of corrosion* 

 The quartz of corrosion generally occurs in small patches in which 

 D ( 33 ) 



