488 Professor J. B. Harrison—‘ Laterite’ in British Guiana. 
Dr. 8. P. Woodward in his classical paper ‘‘On the Structure and 
Affinities of the Hippuritide” (Quart. Journ. Geol. Soc., vol. xi, 
pp. 40-61, pl. v, figs. 1, 2, 1855), and preserved in the British 
Museum (Nat. Hist., No. 38219), obtained from the Iieraster 
cor -anguinum zone of the Chalk at Rosherville, Gravesend, the internal 
chambers have been filled up solid and the extremely thin, shelly 
septa afterwards dissolved away, leaving only an internal mould 
composed of very hard chalk representing some of the chambers of the 
original shell (see Fig. 4). The shelly septa, always very thin and 
fragile, are commonly entirely dissolved out, or, as in this instance, 
replaced by a mould of chalk or flint, the shells of many Radiolites 
being more or less converted into or embedded in flint, as is also the 
case with the shells of Znoceramus from the Chalk. 
Here, then, I think we have succeeded in finding a solution for the 
exceedingly puzzling rolled fossil in flint from the Sherringham Beach, 
and I feel justified in considering it to be the waterworn fragment of 
the chambered portion of the shell of a Radiolite, and most probably 
of FR. Mortoni, replaced by silica. ‘he outer prismatic cellular shell- 
wall in which this chambered portion was originally enclosed (see 
Fig. 5) has entirely disappeared, and the only evidence left is a relic 
of the mould in flint, of the chambers of which we see only the thin 
edges as they pass inwards to form in succession the series of transverse 
floors of what had been at one time a part of the body-chamber of the 
animal. 
III.—Tue Resmpvat Earras or British GursNA COMMONLY TERMED 
‘ LATERITE’. 
By Professor J. B. Harrison, C.M.G., M.A., F.G.S., F.1.C., assisted by 
K. D. Rerp, Assistant Analyst British Guiana. 
(Continued from the October Number, p. 452.) 
({\HE Christianburg and Akyma deposits afford excellent examples 
of the highly aluminous or bauxitic type of laterite formation. 
They here occur as parts of a residuary deposit derived from the 
decomposition in situ of an igneous rock, probably a porphyrite or 
a tuff, an almost pure quartz-sand, and masses of bauxite containing 
in the more aluminous varieties from 92°6 to 94:4 per cent. of the 
hydrates of alumina and in the ferruginous ones 64°6 per cent. of 
the hydrates of alumina with about 24 per cent. of the oxides and 
hydrates of iron. 
The Christianburg and Akyma deposits illustrate excellently the 
extreme of the formation of laterite, where the igneous rock, in place 
of weathering to a mixture of quartz, of kaolinite, of bauxite, and of 
the oxides and hydrates of iron, changes almost completely to quartz 
and to hydrates of alumina and the oxides and hydrates of iron. No 
explanation of the extreme intensity of the lateritic action in the 
Christianburg—Akyma district is at present obtainable. But as it is 
proposed to work the deposits I may be able to obtain further 
information when clean sections of them are available. 
