228 THE PHYSICAL MANIFESTATIONS OF WEATHEEING 



Analyses of Feesh and Disintegrated Oligoclase, Wilmington, 



Delaware 





I 



II 



III 



OONSTITUBKTS 





Opaque White, 



Fine Dust from 





Fresh Oligoclase 



BUT STILL FlEM 



Disintegrated 







Oligoclase 



Oligoclase 



SiOa 



64.76% 



61.23% 



66.73% 



AI2O3 



23.56 



26.65 



28.44 



CaO 



2.84 



2.37 



2.95 



K2O 



1.11 



0.72 



1.12 



NaaO 



9.04 



7.66 



5.81 



Ignition 



* « • • 



1.00 



5.67 





101.30 % 



98.63% 



100.72% 



The fact that granitic and gneissic rocks may undergo ex- 

 tensive disintegration with slight decomposition, even in a 

 moist climate, was noted by Nordenskiold^ in Ceylon. He 

 says: ^'The boundary between the nnweathered granite and 

 that which has been converted into sand ii often so sharp that 

 a stroke of the hammer separates the crust of granitic sand 

 from the granite blocks. They have an almost fresh surface, 

 and a couple of millimetres within the boundary the rock is quite 

 unaltered. No formation of clay takes place and the alteration 

 to which the rocks are subjected, therefore, consists in a crum- 

 bling or formation of sand, and not, or at least only to a very 

 small extent, in a chemical change. At every road section 

 between Galle, Colombo, and Ratnapoora the granite and gneiss 

 crumbled down to a coarse sand, which was again bound to- 

 gether by newly formed hydrated peroxide of iron to a peculiar 

 porous sandstone, called by the natives cabooh.^ This sandstone 

 forms the layer lying next the rock in nearly all the hills on that 

 part of the island which we visited. It evidently belongs to 

 an earlier geological period than the Quaternary, for it is older 

 than the recent formation of valleys and rivers. The cabook 

 often contains large, rounded, unweathered granite blocks, quite 

 resembling the rolled stone blocks in Sweden. In this way 

 there arises at places where the cabook stratum has again 



^Voyage of the Vega, Yol. II, 1881, p. 420. 



^Laterite? It seems so regarded by H. F. Alexander, Trans. Edinburgh 

 Oeol. Society, Vol. II, 1869-74, p. 113. 



