Professor J. B. Harrison—‘ Laterite’ in British Guiana. 443 
Kaolinite,’ whilst the residuary alumina unallotted to them has been 
assumed to be present as aluminium hydrate, the composition of the 
hydrate being dependent on the relative available proportions of 
alumina and of water; and the hydrate so calculated for convenience 
has been termed bauxite—a possible terminological inexactitude which 
I trust will be condoned by Mr. Crook. 
In samples which showed the presence of an excess of water, after 
allowing for the various hydrated aluminous and magnesian minerals 
assumed to be present, the excess of water has been calculated as 
combined with iron peroxide to form limonite. I have not attempted 
in cases where the excess of water is insufficient to combine with all 
the iron oxide, other than that present as ilmenite, to form limonite, 
to show the iron oxide as one of the ill-defined reputed hydrates of 
iron, but have regarded it as being in the form of mixtures in various 
proportions of hematite and limonite. The titanium oxide has been 
shown as present in the form of ilmenite, a mineral very resistant to 
decomposition under the conditions existent in the Guianas, and 
constantly found in the residual earths and the ferruginous and 
aluminous concretionary masses. 
Tue Restpuat Propucts or THE WEATHERING OF ROCKS OF THE 
DraBasE-GABBRO TYPE, 
To ascertain the nature of the residua resulting from the decom- 
position in situ under tropical conditions of basic igneous rocks, 
a study has been made of the various components of such decom- 
position-products at the Agricultural Experiment Station of this 
Department at Issorora Hill, part of the Aruka range, which is 
situated about 17 miles from the seashore, in the north-western 
district of British Guiana, about 180 miles to the north-west of 
Georgetown, and about 10 miles from the Venezuelan boundary, the 
Amacura River. This isolated range of low hills, which extends for 
about 20 miles in a south-westerly direction, consists of epidiorite and 
of hornblende-schist, the metamorphosed products of a diabase or 
gabbro, which are covered by a blanket-like coating of red earth with, 
in its upper layers, numerous blocks of concretionary ironstone, and 
this covering supplies an excellent example of the material not 
unfrequently described as laterite by agriculturists, engineers, and 
mining experts. 
Examination in the field shows that the covering consists of 
a gravelly bright-red earth, similar to that which is termed in Brazil 
terra roxa and in Venezuela cascajo, in which is embedded masses of 
concretionary ironstone, varying greatly in size, in colour, and in 
texture, but usually being ruddy, scoriaceous, cindery, or slaggy- 
looking masses, with innumerable small cavities; large angular masses 
of white quartz, and numerous small pisolitic granules of ironstone. 
Concretionary ironstone of the kind above described is known in 
1 The statement that kaolinite is never a product of weathering but is always due 
to deep-seated changes cannot be accepted for the Guianas, where granitic and similar 
rocks are found converted by surface changes (weathering) into quartziferous kaolins, 
and the various stages of such conversion can be readily followed in the field in many 
places. 
