120 HOLLAND: CHARNOCKITE SERIES. 



next duty is to apply to each group or family so defined the tests 

 which are recognised as safe criteria for distinguishing rocks of 

 igneous from rocks of sedimentary origin. The first section of this 

 memoir is therefore devoted to explaining the general features which 

 Serve to connect and show the consanguinity of the types grouped 

 together under the name charnockite series, whilst the succeeding 

 chapters discuss the question of their origin. 



The most abundant and not the least interesting of the old 

 crystalline formations in South India are the great masses of rock 

 whose two leading characteristics are a granulitic structure and the 

 invariable presence of a rhombic pyroxene amongst the constituents. 

 That such rocks as these existed in the Madras Presidency was, so far 

 as I can find, first recognised by Professor Judd, who employed 

 material collected in the Nilgiris to describe the properties of the 

 highly pleochroic, rhombic pyroxene which approaches Vom Rath's 

 amblystegite in composition. 1 



Later, Prof. A, Lacroix, 2 in his memoir "Contributions a l'etude 

 des gneiss a pyroxene et des roches a wernerite " described as 

 pyroxene-gneisses a number of specimens which had been collected 

 as long ago as 1 819 by Leschenault de la Tour. 3 Some of these 

 rocks resemble the types herein grouped together under the name 

 charnockite series. 



At the end of 1891, and during the first three months of 1892, 

 I made a tour through the southern districts of the Madras Presidency 

 and then found that these rocks had a very wide distribution in the 

 South of India. They were first found in full variety at St. Thomas' 

 Mount and Pallavaram, 10 miles south of Madras city ; from quarries 

 in these localities large quantities of rock have been obtained for 

 building and ornamental purposes in Madras. Subsequently the 

 same rocks were found to make up the mountain masses of the 



1 Quart. Joum., Geol. Soc, Vol. XLI (1885), pp. 371 and 372. 



2 Bull, de la Soc. Fr. de Min., Vol. XII (1889), p. 83, and Rec. Geol. Surv. Ind., Vol. 

 XXIV (1891), p. 157 (translation by Mallet). 



8 In a separate paper I have given an account of M. Leschenault's geological observa- 

 tions in South India, together with the results of an attempt to identify the localities of the 

 specimens described by Lacroix. 



( 2 ) 



