A RAVALLI SYSTEM. 2l 



the calc-gneiss elsewhere, namely calcite, quartz, microcline, diop- 

 side, etc. The idocrase occurs as bands several inches across, 

 exhibiting large shining poikilitic cleavage plates surrounding and 

 enclosing all the other constituents. Thus the rock is not pure 

 idocrase, the percentage of this mineral to the rest being about 5 to 

 1. The specific gravity of the impure rock is 3*31. The mineral 

 has not been chemically analysed as yet (on account of the enclosed 

 impurities this would be a difficult operation). Its determination 



is based on its physical and optical characters, which are as follows : 



hardness about 6-5, dark clear brown colour, vitreous lustre, un- 

 iaxial (some isotropic sections giving a black cross and rings in con- 

 vergent polarised light), refractive index high, double-refraction 

 weak, extinction angle straight with the cleavage and general elong- 

 ation of the crystal areas which are negative. No combination 

 of crystal faces was seen, but the characteristic striated prism faces 

 were observed in one or two places in the rock specimen. I think 

 from the above that there is no reasonable doubt that the mineral 

 is idocrase. Its hardness and other features distinguish it from 

 corundum and its refractive index from gehlenite ; its uniaxial 

 character distinguishes it from zoisite. The locality is at the base 

 of the east slopes of the calc-gneiss hill, | mile west of Nadri, and 

 not far from where the tank (reservoir) dam joins the hill-side. 



Another locality near this, and lying | mile N.N.W. of Nadri, 

 shows the rock (^ft, 12154) composed of a few bands of idocrase 

 associated with quantities of grossular garnet in very small grains, 

 besides other typical calc-gneiss minerals. The grossularite, as is common 

 in contact metamorphosed rocks, has anomalous double refraction. 



From the above account of the calc-gneisses of these areas 



similar calc-gneisses ]t wil1 k° seen that the carbonate, as well as 



from other parts of the silicates, composing them are essentially 



India. | .. ,ii . 



calnlerous rather than magnesian. I have 

 detected no dolomite and none of the more specially magnesia- 

 bearing silicates such as forsterite. brucite, serpentine, and no 

 tremolite. In this respect these calc-gneisses are very similar to 

 those recently described by Dr. Fermor from the Central 

 Piovinces 1 and recently again reviewed by him. 9 They also bear 

 a considerable resemblance to the pyroxene-scapolite granulites 

 and calciphyres of the Salem and Coimbatore districts (MS. 



• 



1 lice, Geol. Surv. Lull,,, WA. XXXIII (1906), pt :!. p. 159. 



- Genl. Rep., Geol. Surv. India, Reo, Vol. XLV, pt. 2(1910), pp. 100-2. 



