No. 5. — 1849.] GEOLOGY OF CEYLON. 213 



laterite, imbedded in a stratum of dark shale and clays. 

 Lignite has also been seen in the laterite of Travancore, 

 and graphite has also been observed there. These are the 

 observations which have made Captain Newbold and others 

 view the laterite of Southern India as a distinct formation, 

 more recent than any of the hypogene rocks. Till similar 

 features are observed in some of the laterites of Ceylon, 

 we are obliged to regard them to be the weathering of 

 hypogene rocks in situ. 



To comprehend how a hard compact rock like granite or 

 gneiss could moulder away into laterite and lithomarge, 

 it is necessary to know the composition of the minerals 

 which enter into the formation of these hypogene rocks. 



The following are the mineral constituents of the most 



common forms of : — 











Felspar. 



Mica. 



Hornblende. 



Silica 



66-75 ... 



48-00 



... 42-00 



Alumina 



17-50 ... 



34-25 



... 12-00 



Lime 



1*05 ... 





... 11-00 



Potash 



. 12-00 ... 



8-75 



... a trace. 



Magnesia 







... 2-25 



Oxide of Iron 



•75 ... 



•50 



*25 



Oxide of Manganese .. 





•50 



•25 



Water 







•75 





98-25 



9600 



98-25 



Quartz consists of nearly pure silica, with a trace, however, 

 of alumina and sometimes of iron.— From Jameisoris Journal. 



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 

 electric 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 porcelain clay or kaolin so abundant on the plains of 



g 2 



