130 HOLLAND : GEOLOGY OP THE NEIGHBOURHOOD OF SALEM. 



mass. As it often does when cutting through the charnockite series, 

 this dyke determines the direction of a depression instead of stand- 

 ing up above the rocks around. On the south-east side it passes 

 through the valley which separates the so-called Twin Hills, and 

 from there it may be traced north-westwards to the Kadiampatti 

 (Atur) ghat where a small stream rushes down approximately parallel 

 to the course of the dyke, and has cut out the depression along which 

 the ghat road from the Kadiampatti railway station to the top 

 of the plateau has been constructed. Whilst this dyke can be traced 

 completely across the plateau from one edge to the other, it is not 

 known beyond the limits of the Shevaroy mass. 



Another dyke, similar in size (that is about 50 yards wide) 

 and in general characters, commences abruptly 



Karipatti dyke. a , r J . 



at a point about one mile south-east of 

 Karipatti, and thence runs south-eastwards. Although these two 

 dykes are similar in size, direction and petrological characters, they 

 are not exactly in line with one another, and no trace of a dyke has 

 been found at any intermediate point. A third dyke has, however, 

 been found much further to the westward, near the south-eastern 

 corner of the larger area of peridotites. This exposure of the rock 

 is about as wide as it is long ; and its edges being covered with 

 talus material, it is difficult to determine its direction. 



All these rocks are augite-diorites (diabases) with micropeg- 

 matite, and in all three exposures they show 

 the essential features of the least basic division 

 of the rocks which in South India are generally regarded as the 

 dyke-representatives of the Cuddapah lava-flows. These Cudda- 

 pah dyke-rocks are divisible into three groups, which, however, are 

 not sharply marked off from one another, but pass by insensible 

 gradations from one to the other, and present throughout a similar- 

 ity of certain peculiarities which indicate that they were derived 

 from the same magma, differing from one another merely through 



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