No. 5. — 1849.] GEOLOGY OF CEYLON. 



211 



clay, tabular or sinuous, in which are impacted quartz 

 crystals of various sizes and colours, generally of a reddish 

 or brick colour. To this kind the term quartzose may be 

 applied, as it contains a larger proportion of undecomposed 

 quartz. The cavities and sinuosities are lined, or sometimes 

 filled with a whitish, yellowish, or reddish clay. 



(2) A second variety of laterite, and that most frequently 

 met with in Ceylon, is of a softer consistence, and can be cut 

 easily with a knife, but hardens on exposure to the atmosphere. 

 The term Uthomargic laterite has been applied to this kind. 



(3) There is another form which my friend Staff Surgeon 

 Dr. Clark calls detrital. This is found in nullahs or 

 ravines. It is evidently formed of pebbles of quartz loosely 

 imbedded in clay, both being washed down to these nullahs 

 by the heavy rains. The detritus of laterite is seen about 

 Colombo forming a braeccia with marine shells. A laterite 

 gravel is also seen in various parts of the Island covering 

 the laterite hills, and it is also found at their base. This 

 gravel is nothing more than the quartz crystals of the 

 laterite rocks separated by the rains from their clayey matrix: 

 some of the pebbles are denuded entirely of the clayey 

 covering, others retain still a thin coating of it. Lithomarge 

 is a sectile clayey substance of variegated colours. It is 

 chiefly formed of a decomposed felspar and hornblende, — 

 whitish when the former prevails, and yellow or reddish 

 when hornblende predominates in the rock from which it is 

 derived, owing to the larger proportion of oxide of iron 

 which the latter mineral contains. There are extensive hills 

 of lithomarge in Ceylon, and frequently it lies under the 

 hard laterite, and is often interposed between its layers. 



With the exception of Voysey and his few supporters 

 (who regard the laterite to be of igneous or volcanic 

 origin), geologists consider laterite to be the product of the 

 disintegration and decomposition of granitic rocks. The 



57—87 g 



