DESCKIPTIO N OF THE CHIEF TYPES. 165 



place whilst the norites were still hot, as no signs of chilled edges 

 are noticeable, anc? this fact, together with the similarity in minera- 

 logical characters between the ferro-magnesian minerals composing 

 the dyke-rocks and those entering into the composition of the asso- 

 ciated norites, point to the genetic relationship of the two rocks 

 notwithstanding the undoubted differences between their periods of 

 consolidation. 



These pyroxenite dykes often show a slight banding as if 

 successive injection had occurred. A case is illustrated by No. 9*672 

 in which bands of pure pyroxene-rock alternate with hornblendic 

 bands without, however, a sharp junction line between them 

 (Slide No. 1438). 



In describing the basic dykes which cut through the char- 

 nockites and other massive rocks in South India I have referred to the 

 occurrence of enstatite-bearing rocks which also approach pyrox- 

 enites in composition, 1 but there is no doubt about the differences 

 between those (which are relatives of the supposed Cuddapah traps) 

 and the pyroxenites now under consideration, which are relatives of 

 the older charnockite series. Although the constituent minerals 

 belong to the same species in both cases the crystal-habits are 

 sufficiently marked to enable a distinction to be readily made even 

 in a microscopic section. If this conclusion be correct we ought to 

 find instances of trap dykes of the kind described in the paper just 

 referred to cutting across pyroxenite dykes of the kind occurring in 

 the Pammal hill and now under description. The absence up to the 

 present of evidence of this kind I prefer to attribute to the limited 

 number of observations hitherto made. 2 



1 Rec. Geol. Surv. Ind., Vol. XXX (1897), p. 30. 



* Mr. Middbmiss has called my attention to two dykes exposed at a point 6 miles south 

 of Royakotta in the Salem District, which in my opinion ought to be regarded as evidence in 

 support of this conclusion. One of the dykes is a pyroxenite similar in character to those 

 described in this paper as ultra-basic members of the charnockite series. It has a specific 

 gravity of 3*31 and is composed of a medium to finegrained mixture of hypersthene, often 

 schillerized, and pale augite with brownish-green hornblende, without white minerals 

 and practically devoid also of opaque iron ores. These characters it will be seen agree with 

 those of the pyroxenites o< Pammal hill near Pallavaram. This rock is cut through by a 

 later dyke which is undoubtedly similar to the rocks generally regarded as the dyke-repre- 

 sentatives of the Cuddapah lavas, and in mineral composition and structure is related tolthose 

 which 1 have described under the name augite-norite {Fee. Geol. Surv. Ind., Vol. XXX, p. 27), 



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