Dr Kelaart's Notes on the Geology of Ceylon. 31 



gneiss, could moulder away into laterite and lifchomarge, it is 

 necessary to know the composition of the minerals which 

 enter into the formation of these rocks. 



The following are the mineral constituents of the most 

 common forms of 



Silica, . 

 Alumina, 

 Lime, . 

 Potash, . 

 Magnesia 

 Oxide of Iron, 

 Oxide of Manganese 

 Water, . 



Felspar. 



66-75 



17-50 



1-05 



12-00 



075 



Mica. 



48-00 



34-25 



8-75 



0-50 

 0-50 



Hornblende. 



42-00 



12-00 



11-00 



a trace. 



2-25 



0-25 



0-25 



075 



98-25 96-00 98-25 



Quartz consists of nearly pure silica, with a trace how- 

 ever of alumina, and sometimes of iron.* 



It is easily seen, that the chief source of the alumina 

 necessary for the formation of clay, is derived from the 

 felspar and mica which enter into the composition of granitic 

 rocks, and that hornblende supplies the largest quantity of 

 iron, the hyperoxidation of which, assisted probably by elec- 

 tric influences, precedes the disintegration of these rocks. 

 In rocks in which felspar and hornblende predominate, the 

 clay formed is much variegated, pure felspar forms the porce- 

 lain clay or kaolin so abundant on the plains of Nuwera Ellia. 

 Quartz, if deeply impregnated with oxide of iron, will also 

 moulder away, but not quite so soon as the other mineral 

 constituents of hypogene rocks. 



Before I had observed the immense lithomargic hills of 

 Ouvah and Nuwera Ellia, it was difficult for me to believe 

 that large mountain masses of hard rock could disintegrate 

 so completely into lithomarge. When there are, however, 

 such unequivocal proofs of rocks, several hundred feet high, 

 mouldering away into kaolin or white porcelain clay in some 

 parts, and in others into lithomargic earths and clays of 

 various colours and consistence, it is not difficult to account 

 even for the formation of the harder forms of laterite. In 

 sections made in Nuwera Ellia for the construction of roads, 



* From Jameson's Journal. 



