450 Professor J. B Harrison—‘ Laterite’ in British Guiana. 
masses resemble in colour and general characters the concretionary 
ironstones described under numbers 5 and 6 of the Issorora District. 
Specimens of this kind were obtained from a shoal in the Demerara 
River near Wismar, about half a mile south of Christianburg. 
The compositions of the samples of the sands, of the bauxites, of 
the underlying clay, and of the hornblende-felspar-porphyrite from 
which they were presumably derived are as follows :— ; 
Tassie VIII. 
3 Bauxite or 
Christianburg. ; 
Hornblende| TLatenite 
Felspar 
Porphyrite. Bauxite 
Sand. or Clay. | Akyma. | Wismar. 
Laterite. 
Quartz. Bi at Reis) es 34:10 96°80 "42 38°79 67 4-74 
Colloid Silica Agente as 14 53 12 62 
Combined Silica . . . 36°86 Hols} 2°29 25°50 1:92 3°58 
Aluminium Oxide... 16°64 “97 67°28 21°16 64°86 45°14 
ImonwReroxidey seen wane 0°22 64 1°53 3-74 85 23°03 
Ino Iara 4, 5 4c 1°48 
Magnesium Oxide. . . ilo) “02 07 “49 “31 31 
Calcimm Oxides = 5 = = 3°46 “08 02 “08 03 trace 
NodrmyOxides 2s... 4°39 “08 nil -48 nil nil 
Potassium Oxide . . . 0°24 “01 “08 2-00 “5 “15 
a WVialbens |p tk. site 0°68 25 27°46 6°67 30°47 21°68 
| Carbonic Anhydride ace 0°42 trace trace nil 
Litanium Oxide .. . 0°38 trace 1:07 67 “TO. |e eluale 
Phosphoric Anhydride. . 0006 "006 | trace trace trace trace 
Manganese Oxide . . . 0:10 trace nil nil nil 
100°266 99:986 | 100°36 | 100:11 | 100°138 | 100°387 
The proximate mineralogical compositions of the porphyrite, the 
bauxitic masses, and of the clay underlying the latter are as shown in 
Table IX (p. 451). 
The bauxitic masses fully correspond to the typical laterites in the 
restricted sense laid down for the late Dr. Buchanan by Mr. Crook. 
They are when first dug fairly soft, and easily trimmed into shape for 
building purposes by a trowel or heavy knife. They have been used 
for building purposes in the foundations and retaining walls of the 
Government Saw Mill at Christianburg, and also in walls at Akyma. 
After exposure to the atmosphere the masses have quickly set and 
attained a hardness corresponding in the lghter-coloured or more 
purely bauxitic parts to somewhat less than three in Mohs’ scale of 
hardness, and in the ferruginous parts to somewhat over that degree. 
The marked hardening is to a considerable extent confined to the 
exposed surfaces. 
I have examined microscopically thin sections of the bauxite masses 
from Christianburg. Their general structure is that of a metamorphosed 
