Professor J. B. Harrison—‘ Laterite’ in British Guiana. 447 
concretionary ironstones are less in evidence than they are at Issorora, 
but in time the weathered surfaces of the earths become covered by 
quartz-sand and gravel, with in places pisolitic granules of con- 
eretionary ironstone, whilst they contain here and there in their upper 
parts masses of concretionary ironstone, some of which attain weights 
of over a ton, and which masses, as are some of those at Issorora, 
are in parts more or less bauxitic. 
TaBLe V. 
Tumatumari. Omai Falls. Mazaruni. 
Diabase. | Laterite.| Diabase. | Laterite. pee ien de Laterite. 
chist. 
@uantzgaes Be 372 47°3 Cron I 887, 16 32°5 
Colloud)Silica 2. OC. ail 2) 
Onihoelase: 3 3s 5 rai % 373 13} 3°9 2°8 25) 2°8 
BineIOease 49:8 2°3 40°6 1:8 30°9 4:5 
Hornblende and Pyroxene 38-0 42°2 53°9 
INGAAS Sop SROs 4-4 6°3 5°6 
iieematite: 0 6) Sk 10:0 12 6°7 
amioniere Git aoa. 7D 
Mente Melee ae el ow iis 0:8 ios} 0-6 9) 0°6 127/ 
Gaolimiters, a vce a es i3} 10°1 20-1 
Salle 51 Ne a ai “4. a3 
1 BTUORIINS). eet th lk 36:0 20°5 31°5 
*Minor constituents. . . 0°5 oy 0:9 
. 100:0 100°3 100°1 100:0 100°0 100°1 
*Diaspore . Ome 24-0 
Gibbsite MAN 30°8 20-5 °5 
*Total Alumina present in 25-0 13°4 25°3 
Bauxite 
As a rule they do not show the hardening properties of Buchanan’s 
laterite, but if we are to take the presence of free alumina in the form 
of hydrate as the definitive test of whether a mass has a right to be 
termed ‘lateritic’, they are clearly lateritic earths. 
The Residual Earths from Serveite and Chlorite Schists— As examples 
of a type of residuary earths which exhibit to a considerable extent 
the setting properties from which Buchanan derived the term laterite, 
may be adduced certain ferruginous earths which contain sericitic 
micas in such quantities that the principal aluminous components 
are sericite, kaolinite, and bauxite, instead of being kaolinite and 
bauxite, with a little residuary felspar, as are those of the types 
already considered. The best examples of these I have seen 
occur at the Omai Gold Mines, Essequibo River, where a ditch 
between two and three miles in length and several tunnels were 
