114 
Rex T. Prider. 
sucli sedimentary rocks ai'e known to the east of the Darling Scarp, 
although there are metasedimentary rocks Avhich could have yielded the 
well-rounded zircons ))ut these rocks (mica schists and quartzites) would 
not yield directly the small Avell-rounded quartz grains since the quartz 
in these rocks lias keen completely recrystallised and in the rock occurs 
as irregular interlocking grains (Prider, P)44, ]>. 1)2). 
(d) The surface (exture of the sand grains of the 10 to 32, 32 to 60, 
and 00 to llo Tyler mesh grades was examined with the binocular micro- 
scope in dry mounts on a dark ground for t!u‘ (piartz griains and on a 
white ground for th(‘ heavy minerals, with the following results: — 
The 00 to 11") mesh grade (Vs to mm.) consists of a]>proxiinateIy 
0”) per cenl. of rough irregular-surfaced gi’aius with polished or fracture 
surface and live per cimt. of smooth-surfaced rounded grains with jiolished 
surfaces Avhicli are often pitted but not frosted. The 32 to 60 mesh grade 
I to V 2 nnn.) contains rough- and siuooUi-surfnced grains in a]>proxi- 
mately equal amounts. The smooth grains are well-rounded Avith polished 
(although somewhat pitterl) surfac(‘s — .some grain> show slight frosting 
and crescentic ]n*i'cussi(ui marks are not uncommon. The rough-surfaced 
grains mostly show slight rounding and are all jjolished or bounded l)y 
vitreous-lustred fracture .surfaces. In the 16 to 32 mesli grade (V 2 to 
1 mm.) there is a high proportion (approximately 90 per cent.) of smooth- 
surfaced grains Avhich vary from dull to polished. Most of these grains 
have a matte appearance due to minute pitting but this is not a frosted 
surface but a|)pears rather to bo the result of a polish superimpAosed 
on earlier frosting. Crescentic percmssion marks are generally well 
developed. 
Twenhofei (194.'), p. 67) considers that frosting may bo developed on 
quartz grains exceeding one mm. diameter on marine beaches but not on 
grains smaller than one mm. whicli can only be frosted by Avind action. 
The aboA’e observations on the surface texture of the <rrains of the Ridge 
Hill ferruginous sandstone therefore are indicative of beach action. 
From these considerations of the mechanical constitution, degTee of 
rounding of the grains and their surface textures it appears most probable 
that the d('trital materials of these ferruginous sandstones and conglom- 
erates Avere deposited on a sea beach and are not tluviatile deposits. The 
basal conglomerate layer represents aecuniulation.s in a narroAv emhayment 
in the coastline existing at this time. The anomalous occirn'ence of 
pebbles and cobbles in the! sandstones is accounted for by the close proxi- 
mity to the east of the Pre-Cambrian landmass which yielded the detrital 
material, these boulders being the result of marine abrasion and having 
suffered practically no transport except on the beach. 
Owing to the ah.sence of fossils the geological ag‘e of the' ferruginous 
sandstone series is indeterminable. It may be either — - 
(a) of Lower (’retaceous (f) age similar to the sandstones and 
leaf-bearing shales of Bullsbrook (Clarke, Prider and 
Teichert, 1944, p. 27o) which is situated on the Darling Scarp 
some 16 miles north from Ridge Hill, or 
(b) later than the formation of the high-level laterite (^Miocene). 
