Professor J. B. Harrison—‘ Laterite’ in British Guiana. 493 
termed laterite, as they neither indurate on exposure to the atmosphere 
nor do they consist mainly or even to any extent of the hydrates of 
alumina. 
Rocks other than highly aluminous ones showing to marked degrees the 
characteristic property of Buchanan's Laterite.—The gradual setting 
or hardening on exposure to the atmosphere of residual masses is 
not a characteristic only of ‘either purely ferruginous, partly 
ferruginous and partly aluminous, or purely aluminous deposits. 
It may characterize to even a more marked degree other residuary 
deposits from igneous rocks. As an instance I may mention a mottled, 
ereamy-white, and dark-red deposit which occurs in a low cliff at 
Kongkamo near the Ireng River in the territory recently assigned 
to Brazil. This is a clay-like mass which when first dug is quite 
soft and can be cut lke cheese by an iron knife.! Upon exposure 
to the air the mass indurates gradually until the creamy-white parts 
attain a hardness of from 3:5 to nearly 4 of Mohs’ scale, the dark- 
red parts becoming still harder. The composition of this material is 
shown by the following analysis :— 
TABLE XY. 
Quartz . : F P : “94 
Colloid Silica . F p : 5 -58 
Combined Silica. : ; J 43°82 
Aluminium Oxide . : ; i 30°49 
Tron Peroxide j : , : 4-40 
Magnesium Oxide . : ‘ : “41 
Calcium Oxide j : : : oy) 
Sodium Oxide ; 5 4 ; 63 
Potassium Oxide . : ; ; 7°53 
Water . : 3 d , ; 5:23 
Titanium Oxide . : : : 60 
Phosphoric Anhydride. : 3 trace 
Manganese Oxide . : : 6 nil 
This corresponds to the following mineralogical composition :— 
Taste XVI. 
Quartz . 1 
Colloid Silica 6 
Orthoclase 44-4. 
Plagioclase 59 
Kaolinite Dial 
Tale 1°3 
*+ Bauxite 19°5 
Ilmenite i199) 
Hematite : 3°8 
Minor constituents . a) 
100 
*Diaspore : - : : ‘ 14:2 
Gibbsite 5 : 5:3 
Total Alumina present in Bauxite 17-4 
Microscopic examinations show that it is a like the ferruginous © 
and aluminous masses of the lateritic earths, an alate or 
1 Tt is thus used by the aboriginal Indians in making images and other carved 
ornaments. 
