558 Professor J. B. Harrison—‘ Laterite’ in British Guiana. 
7'7 per cent. of alumina in the state of hydrate. In the majority 
of the analyses of concretionary ironstones given by Du Bois it is 
not possible to demonstrate the presence of free aluminium hydroxide, 
but his analyses of an alluvial one show that at least 5:2 per cent. of 
alumina is present in the form of hydrate. 
The fact that bauxite or gibbsite is found in the laterites of Brazil 
and of the Guiana has been known for many years. R. Hermann, in 
1869 (Journal f. prakt. Chemie, vol. i, p. 72), described hydrargillite 
which occurred in the laterite of Villa Ricca in Brazil, whilst its 
occurrence in French Guiana was recorded in 1878 by Jannetaz in the 
Bulletin de la Société Minéralogique de France, vol. i, p. 70. The first 
full description of typical bauxitic laterite from Surinam is that given 
by Du Bois, pp. 84-7 of his monograph on the laterite of that colony. 
He there gives the following ‘ technical’ analyses :— 
Taste XXIY. 
x XI XII 
Alumina . 63:3 48°5 §2°5 
Tron Peroxide 10°5 21°6 14°4 
Silica : i 14°45 ee Sil 
Calcium Oxide . 2 ; 1 1 1°5 
W ater ; : : ‘i 17°6 14 27°6 
99-4 99-6 99-1 
Du Bois specifically states that the silica is present in them in the 
form of secondary chalcedony. If all of it is so present the bauxite 
contains from 48.5 to 63°3 per cent. of free alumina or from 62°5 
to 80°9 per cent. of aluminium hydrate, whilst the lowest pro- 
portion of alumina that can be present in the uncombined state on the 
assumption that all the silica is combined with alumina will vary from 
40 to 57 per cent. 
The studies which have been made in the Guianas by Du Bois, 
Levat, Lungwitz, van Cappelle, myself, and others, both in the field 
and in the laboratory, show that certain residual deposits derived 
mainly from rocks of the diabase-gabbro, diorite, and porphyrite types 
which contain predominantly felspars of the albite-anorthite series 
have full right to be termed laterite both in the restricted sense of the 
term advocated in this journal by Dr. Evans and Mr. Crook and in 
the wider meaning originally ascribed to it by Buchanan; that masses 
of bauxite occur in residual earths of complex composition which 
cannot be described as characteristically aluminous, as they are usually 
to more marked extents siliceous or ferruginous, although they in- 
variably contain more or less hydroxide; and that the concretions in 
the residual earths vary from concretionary ironstones to impure 
ferruginous bauxite.’ 
I have only seen the laterite in one district of Surinam where it 
formed a covering to a greatly metamorphosed sericitic schist. Here 
there is a gradual change of the decomposing schist into its laterite, 
the change extending to considerable depths in the schist. The 
lateritic earth, covered in places by concretionary masses of ironstone, 
1 In the Guianas the formation of masses of bauxite appears to be due to later 
segregation changes in the metasomatic residual earth rather than to the completeness - 
of the original decompositions. 
