Professor J. B. Harrison—‘ Laterite’ in British Guiana. 489 
The foregoing examples represent the types of the residual deposits 
in situ in British Guiana, which I have regarded as and termed 
laterite. The concretionary masses in various parts of them range 
from ironstones with in round figures 93 per cent. of iron-ores and 
with less than 2 per cent. of aluminium hydrate to bauxites with less 
than 2 per cent. of iron-ores and with over 94 per cent. of aluminium 
hydrate. But the main bulk of the residual deposits in situ correspond 
to those which G. C. Du Bois in his monograph termed “ Eluviale 
Laterite (primaire Laterite)’’, or ‘‘ Kieselsdurereiche Laterite ” 
They are essentially buff, ochreous, red-brown or chocolate-coloured, 
ferruginous, more or less siliceous, clays and earths; and consist of 
mixtures in various proportion of angular quartz-sand and siliceous 
grit derived in part from quartz originally present in the rock of 
which they are products and in part secondarily from the decom- 
position of the felspars; of kaolinite, of bauxite, and of limonite, 
or other more or less hydrated oxides of iron, derived from the 
decomposition, hydration, and oxidation of the constituents of the 
felspars and of the ferro-magnesian minerals. 
The mean proximate composition of the British Guiana lateritic 
earths which I have examined is in round figures :— 
TABLE X. 
Quartz . 5 3 : 3 5 94 
Tron-ores (including pisolites) : ; 32 
Kaolin, Sericite, and other felspathie debris, ete. : 24 
Bauxite . : c : : 2 ¢ : 20 
100 
The earths I quoted as examples of the lateritic earths in my work 
on the Geology of the Gold-fields of British Guiana by weathering, 
detrition, washing, and re-arrangement of their proximate constituents 
could give rise to deposits consisting mainly of quartz-sand and ot 
more or less bauxitic masses. For instance, the Tumatumari earths, 
if separated by natural elutriation, would give rise to a quartz-sand, 
say with 90 per cent. of quartz, and to bauxite with 63 per cent. of 
aluminium hydrate and 20 per cent. of iron-ores. 
Whilst in the examples discussed there is no difficulty in tracing 
the silica set free during the decomposition of the rock into the form 
of masses of quartz as at Issorora, or into that of quartz-sand as in 
the earths from Tumatumari, Omai, Mazaruni, and Christianburg— 
Akyma, there are other cases in which little of the silica remains 
through the mass of the laterite. In some of these the ferruginous 
or aluminous lateritic earths are traversed by numerous thin veins of 
secondary quartz which intersect one another in all directions. These 
quartz-veins, which towards the surface of the laterite are not 
infrequently auriferous to paying extents or to even well-marked 
degrees, lessen in their contents of gold and tend to become barren 
when followed to a depth where they gradually thin completely out 
before reaching the less altered layers which are just above the 
undecomposed rock. In other places, as I have described elsewhere, 
the silica has segregated out in the form of lenticular masses not 
uncommonly described as ‘ quartz-reefs ’. 
