492 Professor J. B. Harrison—‘ Laterite’ in British Guiana. 
microscopical examinations of them show that sericite is present in 
them to a considerable extent. The chemical compositions of two 
typical specimens are as follows :— 
TasLe XIII. 
Minnehaha, Mindrinetti, 
Potaro District. Surinam. 
Quartz ; é : ‘ , Boo 20°30 
Colloid Silica : : : ; “40 38 
Combined Silica . ; : : 32 26°28 
Aluminium Oxide . : ; : 31°49 34°10 
Tron Peroxide ; : 5 : “68 6°56 
Magnesium Oxide . ; . , “21 Oi 
Calcium Oxide . , ‘ : 10 “02 
Sodium Oxide ; ; , : oD) 39 
Potassium Oxide . : ; , Woe Oe 
Water : : : A - 9°67 8:43 
Titanium Oxide . : ; . 67 1 
Phosphoric Anhydride . ( : nil nil 
Manganese Oxide . : 6 ; nil nil 
99°95 99°70 
Their proximate mineralogical compositions appear to be— 
Taste XIV. 
Minnehaha, Mindrinetti, 
Potaro District. Surinam. 
Quartz : 4 ; ; : Deiow7 20°3 
Colloid Silica , j +4. 4 
Sericite 14°3 21:6 
Kaolinite 53:9 34:0 
Tale Or 9 
*7 Bauxite 6:0 15:0 
Ilmenite ios} 2-0 
Hematite 5:5 
Minor constituents » °3 
100 100 
*Diaspore 371 12°8 
Gibbsite : ‘ : : 2°9 DoH) 
{Total Alumina present in Bauxite . 4°6 12:3 
These earths are mixtures of fine, sharply angular quartz-sand, 
kaolinite, and sericite, with lesser proportions of bauxite, the latter 
being present to the extent of 6 per cent. in the Minnehaha sample 
and to 15 per cent. in the one from Surinam. The principal 
differences between the Muindrinetti sample and the Omai sericitic 
laterites already described are in the higher proportion of limonite 
present in the latter and in the higher state of hydration of its 
bauxite. The more highly hydrated rock shows the setting power 
of true laterite; the rock of lower hydration does not in any way 
exhibit it. 
Both these sericitic earths are more or less auriferous, and have 
been worked for gold. 
Neither the pipe-clays, the products of the decomposition of 
granites, pegmatites, more or less aplitic granitite, and aplite in situ, 
nor the sericitic earths, their alteration products, have any claim to be 
