The Geology of the Darling Scarp at Ridge Hill. 
113 
material be disregarded it will he seen that the actual detrital material is 
well graded, 92 per cent, lying between Yg and % mm. diameter. 
(b) Heavy mineral analyses Avere made of the three finest grades, the 
material passing 250 mesh being separated by centrifuging. The light 
fractions consist entirely of (|uarix with the alumina-silica (allophanoid) 
mineral. The hea\'y fractions were further se])arated into magnetic ‘okI 
non-magnetie fractions, the magmdic fiaction (largely ilmenite) h\ each 
grade forming approximately 75 per cent, of the heavy fraction. The 
heavy minerals idiuitified are recorded in Tal)le V and of these zircon is the 
most abundant of the non-o}mc|nes and is w<n‘th further mention as theia* 
are two distinct varieties present, the predominant type in both the 60-115 
and 115-250 mesh fractions biung perfectly rounded and colourless, the other 
tyj)e being slightly Avoni to ]Mwfectly (‘uhedral colonrh'ss to purplish zoned. 
This is indicative of derivation of the detrital material from Iavo diO'erent 
parent rocks such as an igneous rock (e.g. granite) to yield the euhedral 
zircons and a sedimentary or metasedimentary rock to yield the well-rounded 
zircons {roundness 0.9) which have nndonUteflly pa.s-^ed thi’{>ugh move than 
one cycle of erosion. The association of these two typ(*s of zircon may in- 
dicate derivation in the one cycle of erosion from a distributive province 
of the nature of the present Toodyay area (Prider, 1944) whicli is situated 
in a belt of igneous and metasedimentary Pre-Cambrian rocks lying some 
2)0 to 40 miles inland from the Darling St-arp. 
(e) Visual projection roundness (Krumbein, 1941) and sphericity 
(Rittenhouse, 1943) values were determined for tlu^ light fractions of the 
30-32, 32-00, 00-115, 115-250 Tyler mesh grades. The results (shown 
graphically in column 1) in tlgnres five and six) indicate that 
the average sphericity in all fractions is a])proximately the same (0.S3) and 
that the degi'ee of rounding decreases with ileereasing size but there is still 
appreciable rounding of some grains down to 0.124 nun. Oiaineter. During 
the roundness analysis and suliseipient examination of the surface texture 
of the grains it was e\'ident that tiuwe are two distinct types of (punlz sand 
grains i>resent — a well-rounded set and another the grains of which are 
much more angular although still showing considerable abrasion. The pro- 
portion of Avell-rounded to poorly-rounded grains increases with increasing 
grade thus : 
Grade ... 
•124- -246 
•246- -495 
■495- -991 
mm. 
mm. 
mm. 
% of well-rounded grains ... 
... 
5 
50 
90 
The occurrence of a small in-oportion of Avell-ronnded grains in the 
Vs to Vi mm. grade seems indicative of the derivation of the detrital 
material from s(‘voral different sources. The high i)ro]iortion of Avell- 
rounded grains in tin* PS to 1 mm. grade indicates, however, very con- 
siderable abrasion during the last cycle of erosion and since this rounding 
is well marked doAvn to the giaiins of Vi mm. diameter it mii’^t be assumed 
{following TwenJiofeJ, 1945, p. (hi) that this final stage of abrasion must 
have taken plact' on a sea beach. The smaller well-rounded grains may 
he due to an admixture of some aeoliaii-transported sand with the beach 
sand, or may have l)Gen derived from some pre-existing sediment. Xo 
