Professor J. B. Harrison—‘ Laterite’ in British Guiana. 449 
Taste VII. 
Quartz . ; Oiler 
Colloid Silica . =) 
Sericite ; 16°9 
Hematite 2°6 
Limonite 18:6 
Ilmenite . 2:9 
Kaolinite. 23°0 
Tale 7 
*+Bauxite . 13°5 
100°1 
*Gibbsite . : : ; : : 13°5 
+Total Alumina present in the form of 
Bauxite : : ; 3 8:9 
The surfaces of the residual earths are covered with ironstone, 
eravel, and conglomerates, whilst at depths of some feet from the 
surface layers of angular quartz gravel occur. In many places the 
earths are seen to be traversed by numerous thin veins of quartz, 
some of which are normal to the original schists, whilst others are 
clearly of secondary origin, their silica having been derived from the 
decomposition of the felspar and ferruginous minerals of the rocks. 
Layers of ironstone conglomerates and masses of concretionary 
ironstone similar to those at Issorora everywhere cover to a depth of 
about 25 feet the surface of the hills at Omai. Like those at Issorora 
the concretionary ironstones are in places more or less aluminous. 
Laterite from Felsite or Porphyrite, Demerara River.—At Christian- 
burg, on the Demerara River, about 58 miles south of Georgetown, 
where the Government has a Para rubber experiment station, the 
surface of the low hills is a sandy soil which rests on beds of cream-' 
coloured, reddish-grey, or red highly aluminous laterite or ‘ bauxite’. 
The bauxite is also exposed in shallows in the bed of the river between 
Christianburg and Wismar, and it gives rise to a low hill at Akyma, 
some 11 miles south of Christianburg. Microscopic examinations of 
the bauxite show that it has been derived from a felsite, a porphyrite, 
or possibly a tuff. Felsite and porphyrite are exposed in the Kumaru 
Creek close to Akyma Hill, where they are intrusive in gneiss. At 
Christianburg the bauxite apparently is underlain by a pale buff- 
coloured arenaceous clay. 
In this district generally as well as at the Christianburg and the 
Akyma Hills the surface soil over the lateritic decomposition-products 
is a sand varying from white, glistening, almost pure quartz sand, 
containing over 96 per cent. of quartz, to a brownish highly arenaceous 
soil, with from 40 to 50 per.cent. of quartz, the former variety 
containing less than 34 per cent., the latter from 2 to 6 per cent. of 
free alumina, whilst below it the subsoil contains less quartz—from 
20 to 45 per cent.—and somewhat higher proportions of free alumina, 
these ranging from 5 to about 7 per cent. 
The soil is underlain by a somewhat thick deposit, the depth of 
which has not been determined, of massés of cream-coloured bauxite, 
which in parts are more or less stained with iron, or where reddish 
grey to red in colour are more or less ochreous or limonitic. Some 
DECADE V.—VOL. VII.—NO. X. 29 
