ARAVALLI SYSTEM. 11 



exposures till they rather suddenly end near Kherod against the 

 somewhat different Kherod series of amphibolites, and again in 

 the opposite direction near Waken, where they appear to pass into 

 the biotite-gneiss. 



The calc-gneiss here described contains much of the carbonate 



in the form of calcite, as well as several of 



The term " calc- ^ e typical calcium-bearinu silicates, which 



gneiss. J L 



latter are ordinarily sufficient along with other 

 characteristics of such gneisses to allow the prefix " calc " to be 

 applied to them. Very often the rock is a coccolitic marble, similar 

 to the classical British example of the Tiree marbles ; but, as other 

 parts of the great series contain much less of the carbonate and 

 would not ordinarily be alluded to as limestone or marble, I prefer 

 to speak of the rock generally as a calc-gneiss. 



It must be understood to be a rock very well banded in layers of 



varying composition. Hence it appears to possess 

 Banded character. , . ■, -, • ■ ■, , ••> ,. 



what may have been original stratification 



(though there is no real evidence of this) in well-marked beds, 



usually of white, grey and dark greenish grey colours, weathering 



much darker and sometimes almost black. Some laminae under 



surface influences become deeply eroded relatively to the associated 



lamina), and give rise at the surface to those extremes of relief 



and corrosion always so characteristic of metamorphic calciferous 



and scapolitic rocks — as for instance the well known calc-gneiss of 



the Coimbatore neighbourhood in Madras. 



It is constantly and often very closely permeated by aplite 



veins of from a foot or less in width to some- 

 Permeation by aplite j ag much ag m feef and e _ 



tionally to even greater thicknesses (see PI. 

 1, fig. 1). These veins of more or less acid rock will be described later 

 (see p. 31), but it may here be remarked that they generally 

 follow the original banding (? bedding) of the calc-gneiss with 

 occasional transgressions, and that the two rocks thus in com- 

 bination are frequently found together in any fairly continuous 

 outcrop — so completely has the permeation of the one rook 

 by the other been carried out. Notwithstanding the above, the 

 two rock bodies have every appearance of being in their essentials 

 entirely distinct, the one being a typical banded gneiss, and the 

 other apparently a sharply contrasting granite vein or aplite, 



veins. 



