22S HOLLAND: CHARNOCKITE SERIES. 



more accustomed to deal with when an igneous magma intrudes into 

 a cold rock and reveals the fact by showing chilled selvages. 



The conclusions as to the igneous origin and intrusive habit of the 

 _, charnockite series, based on the rather slender 



Dykes with chilled 



selvages. evidence of these trespassing protrusions in 



the neighbourhood of Salem, receive very material support from obser- 

 vations which, in company with my colleague Dr. T. L. Walker, 

 I have recently made in the province of Coorg, on the eastern 

 slopes of the Western Ghat range, where actual chilled selvages have 

 been observed in dykes of rocks belonging to the charnockite series. 



In the north-eastern portion of Coorg the fundamental rocks are 

 banded, crushed and well foliated gneisses, chiefly biotite-gneisses. 

 Between Somwarpet and Jambur the chief rocks are, however, mem- 

 bers of the charnockite series, forming apparently a great mass which 

 is fringed on the south-west and on the north-east sides by bands 

 of the same series. Although these bands are sharply defined, they 

 run parallel to the foliation of the biotite-gneiss in which they lie, 

 and on this account are easily distinguished at once from the 

 younger systems of basic dykes which cut the gneisses without 

 regard to the foliation. On account of the constant way in which 

 these dark bands follow the foliation of the gneisses, I at first 

 regarded them as further instances of the puzzling, but common, 

 cases of interbanding so often observed in the old gneisses ; at the 

 same time their compact nature and jointed condition give them 

 very much the appearance of basic trap dykes. Under the micro- 

 scope they were seen to be composed of the typical constituents of 

 the charnockite series, and were often found to be garnetiferous, and 

 to possess a granulitic structure with the peculiar water-clear, but 

 badly twinned, plagioclase which seems to characterise the members 

 of this series. Subsequently, in a well exposed section of some 

 four or five of these bands cutting through the biotite-gneiss in the 

 bed of the Cauvery river, Dr. Walker detected the compact nature 

 of their selvages. Microscopic examination of these selvages 

 ( no ) 



