CLASSIFICATION OF THE ROCKS. 12Q 



formed by the pinching out of a single band, but more probably are 

 the result of intrusion between the folia. Further masses of 

 pyroxenite are exposed in Nagaramalai (11*904, 11*914) the upper- 

 most rocks of which are precisely similar to those ot the hill west-south- 

 west of Salem. About half-way up the south-western slopes of the 

 Shevaroys there are large masses of these rocks in which horn- 

 blende greatly exceeds the pyroxene in quantity. These rocks are 

 essentially the same as the pyroxenites of Pallavaram (Nos. 9*394, 

 9*667, 9*672, 8*756) and the hornblende-pyroxenites or amphibolites 

 of Tiruppur (9*309) and South Arcot (9*809). The ultra-basic forms 

 —pyroxene and pyroxene-amphibole rocks— so abundant in the 

 neighbourhood of Salem do not appear to have been described by 

 Lacroix, although they are always associated with, and are certainly 

 related to, the basic pyroxenic gneisses. Most of them contain small 

 quantities of olivine and a green spinel. Details concerning their 

 microscopic characters will be found in the separate paper on the 

 charnockite series. 



(4) Younger igneous intrusions. 



The igneous rocks, which appear to be quite independent of 

 the direction of foliation, and which were therefore probably intruded 

 after the folding movements in South India had ceased, form three 

 groups, whose relative ages cannot be determined with certainty 

 for the reason that they do not come into contact with one another. 

 These are — 



{a) Augite-diorite (diabase) dykes with micropegmatite. 



(b) The magnesian series of the " Chalk hills. " 



(c) The " White Elephant rocks". 



4a. — Augite-diorite (diabase) with micropegmatite. 



The most prominent of the basic trap dykes in the area 



under discussion runs in a north-west — SOUth- 

 She varo) dyke. 



east direction completely across the Shevaroy 



( 27 ) 



