16 
Gosnells to Mundijong, at least, there appear at intervals 
exposures of slaty rocks, dipping at high angles, and apparently 
surrounded completely by eruptive materials/ In places, also, 
enormous quartz reefs, or sections of one great quartz reef, are 
met with (Brunswick, Gosnells, etc.). 
Ibis exposure of relatively fresh and undecomposed basement 
rocks bespeaks the rapid erosion which is going on throughout the 
foothill zone. 
(d.) At several points along the outer part of the foothill zone 
there appears a laterite-covered shelf at a very strikingly uniform 
elevation of about 200 leet above sea level. Tt appears, rather ob- 
scurely, at Greenmount and Armadale, much more decidedly at Ridge 
Hill, the lower part of the Kalamunda Road, and Waroona. The ap- 
pearance of this laterite at all points, and its relationships at Ridge 
Hill and Kalamunda, seem to distinguish it from the widely-dis- 
tributed detrital laterite described hv Simpsonf as occurring at 
low levels. Much more research will have to be carried out before 
the existence of this shelf as a definite independent element can he 
claimed. Tentatively, however, we may suggest its existence and 
explain it as the remnants of a step-faulted portion of the plateau, 
let down by a subsidiary fault, immediately to the west of, and par- 
allel to the main Darling Range fault. 
For this somewhat hypothetical element 1 suggest the name 
“Ridge Hill Shelf/ 
(e.) We reach now the main “Darling’ Fault” which has been 
responsible for the entire structure of the region, and which separ- 
ates the isostatically adjusted uplift area on the east, and subsidence 
area on the west. The actual fault plain is not known to the author 
at any point, the line of junction of the ancient crystalline rocks 
of the plateau and of the recent accumulations of the plain being 
hidden by detritus. Of the existence of such a fault, however, there 
can be no doubt, and of its tectonic importance there can lie no 
question. 
(/.) The “Piedmont Zone” follows next in order and is of the 
utmost economic importance. Steep-grade, rapid streams flowing 
down from the youthful valleys of the plateau element bear with 
them the well rotted detritus derived from both granites and “green- 
stones.” On reaching the plains these streams have their velocity 
checked and are forced to deposit their sediment as flat alluvial fans 
or dry deltas. The streams are legion, and deposits of adjacent streams 
uniting at their lateral edges, build up an almost continuous, gently 
sloping zone of alluvial matter right along the base of the main hiil 
feature. Containing as it does a modicum of potash derived from 
the felspars of the granites, and quite a notable amount of lime ob- 
*Honman, C. S., The Extension of the Kelmscott Clay Deposit. Bull. Geol. Survey 
W.A., No. 48, 1912, pp. 63-65. 
t Simpson, E. S. : Laterite in Western Australia. Geol. Mag 1 , n.s. rlec. v. vol. ix., 
p. 399-406, 1912. 
