Professor J. B. Harrison—‘ Laterite’ in British Guiana. 451 
felspar - porphyrite or tuff in which the original minerals with the 
exception of the ilmenite have been impregnated with or replaced by 
hydrates of alumina in an amorphous concretionary form. The 
amorphous matter is mingled here and there with very minute 
particles of quartz, and it is traversed by very thin veins of chalcedonic 
silica, some of which appears to be tridymite. The ilmenite is present 
in the form of widely, though sparsely, scattered exceedingly minute 
Tapre IX. 
Ae Bauxite or 
Christianburg. ; 
Hornblende | HiyeSitse 
Felspar | 
Porphyrite. | Bauxite 
Sand. or Clay. | Akyma. | Wismar. 
Laterite. 
Quartz . bit eee ee 34-1 . 96°8 4 38°8 37 4°7 
Colloid Silica : “TL 5 Tl 6 
Orthoclase 1:4 trace! oo) 11:8 “9 9 
Plagioclase unas 51:0 od sal 4-4 
Hornblende and Pyroxene 12:0 
Magnetite a tees 
Heematite 5 oa hee tac gee 6 b) onl oj 21°9 
WMGeMGe! Sieyy ek eo “§ PPM 1:3 1-4 2°1 
IRCAOMIMILCR Nene ee, ied 3°3 29°8 5 4°3 
Tale aes ene of) 1:6 10, 1:0 
Calettcvmmere sf. “9 trace trace 
I AUNIteN so. age “4 92°6 8-9 94°4 64°6 
*Minor constituents . an ye op) 
100-2 | 100-0 | 100-0 | 100-2 | 1¢0-1 | 100-1 
“IDMER ORE 6 Te) 1G Sesy toes 26°71 
Gibbsite: 6051. “4 66°5 2-0 Weil 6°6 
*Total Alumina present in a 65°7 69 82°3 58:0 
Bauxite ae 64:1 43°0 
1 Muscovite. 
grains, none of it, however, being in the siliceous veins. The thin 
sections examined were cut from some of the more siliceous parts of 
the bauxite. The microscopical structure of the bauxite closely 
resembles that of the ‘‘ Oolithartige Bauxit’’ of Surinam, described 
by Du Bois as follows on pp. 35-9 of his monograph “‘ Beitrag zur 
Kenntnis der Surinamischen Laterit’’ (Zschermak’s Mineralogische und 
Petrographische Mitteilungen, Band xxii, Heft 1, 1908) :— 
‘“‘Only in a very thin section of the fresh, oolite-like substance 
could the separation of secondary silica be demonstrated with certainty. 
The ground-mass of the nodules and the cement is composed of feebly 
translucent yellow substance apparently amorphous. Some of the 
concentric parts—generally the outermost—are strongly impregnated 
with microscopic granules of chalcedony. The intermediate mass or 
