ROCKS ANl> MINERALS OF CEYLON. 



41 



twent} -five feet. Numerous minute shells and fragments of 

 shells occur in this. At Pamunugama, on the way to Negombo, 

 the sandstone varies much. In some cases it is black-banded, 

 with particles of magnetic iron ; in others the particles of iron 

 are evenly distributed throughout the mass. The nature of the 

 cement which binds these particles together is carbonate of lime. 



Calcareous Tufa, still in the course of formation, is a 

 deposit from the hot-water springs of Bintenna, the water of 

 which is highly charged with carbonate of lime, which is 

 deposited as the water cools. It is known as Pennagal by the 

 Sinhalese, and is burnt and used by them along with their betel. 



The foregoing formations are indicated on the accompany- 

 ing rough geological sketch map. 



I will now consider more particularly the gneiss, which is 

 our most extensive formation. It varies much in texture, color, 

 composition, hardness, &c. 



Its composition is the same as granite, only the degree of 

 metamorphism has not been so great as to entitle it fully to 

 that name. 



It is composed of quartz— felspar (both orthoclase and 

 oligoclase) — muscovite and biotite (micas), hornblende, 

 chlorite, and numerous accidental minerals in varying pro- 

 portions. 



In some localities we find a rock composed of only one of 

 these, as in the case of hornblende rock ; at other times only 

 felspar, but generally the foregoing components are mingled 

 together in varying proportion^ giving a large number of 

 different kinds of rock. 



Orthoclase forms the main mass. Tlie two fel soars are 

 easily distinguished on a weathered surface* The orthoclase is 

 glossy and somewhat pearly in lustre, and has a translucent 

 aspect, while the oligoclase is dull and opaque. 



In „the gneiss we meet with various beds as limestone, 



