CRYSTALLINE ROCKS OF (JKYLOX. 105 



THE CRYSTALLINE ROCKS OF CEYLON, 



By a. K. Coomaraswamy, B. Sc, 



Direfior, Mineralogical Surrey of Ceylon. 



npHE following Paper is intended to give a brief account of the 

 -*- crystalline rocks of Ceylon, so far as at present known and 

 studied. 



The first question which arises is one of nomenclature. For a 

 long time the rocks of Ceylon were somewhat summarily 

 classified as gneisses, or as belonging to a " metamorphic series.'''' 

 It must here be pointed out that on the Continent the term ^^ gneiss " 

 is often used in its strict original sense and applied only to foliated 

 crystalline rocks having the mineral composition of granites, and 

 thus consisting essentially of orthoclase-felspar and quartz, with 

 one or more of the minerals mica, hornblende, or augite. In this 

 strict sense the term " gneiss " is only applicable to certain of the 

 rocks of Ceylon. It has, however, been widely used in England 

 and even abroad to designate rocks characterized rather by the 

 possession of a " gneissose " structure than by any particular 

 mineral composition. In this extended sense the term is 

 applicable to many or most of the rocks of Ceylon. 



The name '•'■ granuUte''^ has been extensively employed to 

 designate foliated crystalline rocks of fine grain in which ortho- 

 clase, quartz, garnet, and more or less biotite, hornblende, or 

 augite occur, but which are especially characterized by the 

 presence of garnet. 



This term is thus particularly applicable to the majority of 

 Ceylonese rocks, and its use is perhaps to be preferred to that of 

 the term " gneiss." 



Mr. Holland has recently named the rock series of Southern 

 India (which closely resembles that of Ceylon) the Gharnockite 

 Series. Ceylon and Southern India form together a well-marked 

 petrographical province, and there is no doubt that the rocks of 

 Ceylon must be considered as forming part of the charnockite 

 series. It should be noted that the term " charnockite " is used 

 to designate both a whole group of rocks and also one of the types 

 of rock (having particular and definite characters) characteristic 

 of the group. It is thus safest as a general rule to speak of 



