The Geology or the Darling Scarp at Ridge Hill. 
IIT 
(ii) The low-level laterite oeeurs as a thin discontinuous layer above 
the ferruginous sandstones on the Ridge Hill Shelf. H has, on the exposed 
surface, a somewhat fragmental appearance but on breaking the rock these 
fragments are seen to be of fen'uginous sandstone identical in character 
with the underlying ferruginous sandstones which have b('en described above. 
These fragments in a tyi>ical s[)ecimen (21d59) which has been analysed 
average five mm. <liameler JUul they art' coate<l with a dense layer of light 
brown, fine-grained and compact bauxitic material and the sluices between 
the fragments are largely lilled with this hauxilic material but some cavities 
remain giving the rock a slightly cellular structure. This suidace lateritic 
ci’n<t passe^' down into the normal ferruginous sandstone at three or four 
feet below the ground surface and there can be no doul)l that the laterite 
has formed in situ from the sandstones whicii in turn overlie the Pre-Cani- 
hrian granitic rocks. These granites where\'er extiosiul below the ferruginous 
sandstones (e.g. at 45 chains north from Ridge Hill siding) are seen to he 
highly weathered (kaolinised) in the same way as the granites under the 
high-level laterite (the relationships of these rocks are illustrated in text 
tig. 3). 
Granite I h- 1 Loin-level laterite 
[p/diorite Ferruginous sandstone EI 3 
Text fig. 3. — K.-W. section at 33 chains Xorth of I?i(lge Hill siding showing 
relationship of the I»re-rainl)rian granites and epidiorites, ferruginous sand- 
stones and overlying low-level laterite, and tlie younger yellow sands. 
An analysis of tin* Ridge Hill low-level laterite is given in Fable Ir 
analysis 11 where it is compared with the analysis of the underlying fer- 
ruginous sandstone (anal. I) and the high-level laterites (anal. Jll and )■ 
It ditfers from the high-level Interites in its much higher silica content, due 
largely to the presence, of abundant water-worn sand grains residual from 
the ferruginous sandstone from which it was deveIo])ed, but also in part to 
a higher proportion of* combined silica. Comparing the composition of tae 
low-lev(‘] laterite with the luiderlying ferruginous sandstone the most notable 
feature is tlie marked increase during lateritisaiion of the A l.^.Oy I e„0^ ratio 
and the develojmient of the hydrated oxides such as limonite and bauxite. 
In a consideration of the chemical changes sustained by the parent rock 
during the lateritisation i>rocess the only fact{)r which may with some degrip 
of certainty be likely to remain constant is the (|uartz (free silica) content. 
In Table IT the analysis of the laterite (column 2) has been recalculated 
to quartz = 58.51, i.e. these figures would then represent the number of 
oTams of each constituent in a volume of the laterite which contains 58.5 
ovams of quartz. Comparing these figures with those of the juiront fer- 
ruginous sandstone (column 1) the gains and losst^ in the vailous con- 
stituents per 100 grams of the original sandstone may l)e determined 
(column 4). 
