108 
Rex T. Pridee. 
Range area the basic dykes are lesn resistant to erosion than the adjoining 
granite and not more resistant as indicated by Auronsseau and Budge 
(1921 p. do) and Clarke and Williams {1926, ]>. 167), but at the shme 
time they have contact metamorphosed the adjoining granite slightly thus 
rendering it more resistant to erosion than the unaffected granite at some 
distance from the basic dykes, thus accounting for the anomalous behaviour 
of the less resistant l)asic dykes forming the ridges. This observation 
of the relation of t( 4 )ography to the less resistant dykes is of some import- 
ance in geidogical mapping in the Uarling Range area — if shallow gullies 
or dei)ressions are examiued more closely it will generally be found that 
the 'underlying rock is edther basic dyke rock or else sheared granite. 
The Darling Plateau capped by the high-level laterite is exposed in 
several outliers of the plateau in the south-east corner of the area. These 
outliers are flat-topped and surrounded by breakaways (Clarke, Pridei* 
and Teiehert, 1944, ]>. (iO). 
The Ridge Hill Shelf forms almost the entire western part of Ihe map- 
ped area. To the north-west of Kidge Hill siding it is laterite-covercd 
at an elevation of 2o0 feet above sea level and from here it slopes down 
gently and uniformly to the west Avhere it passes eventually into the liat 
eoasal plain. It is immaturely dissected in the nortli-Avest part of the 
area by north-liowing tributaries of tlic Helena IHvei’ which flows almost 
parallel to the northern boundary of the mapped area and at some 10 to 
20 chains to the noi’th of it. 
As noted above, AVoolnough considers this shelf to be the top of the 
downfaulted laterite-eapped Darling IMaleau but evidence Avill be put for- 
ward later in this paper vvhieh indicates rather that this shelf is actually 
an erosion feature such as a wave-cut bench and bears no relation to the 
Darling Plateau. 
Clarke and Williams (1920^ p. 167) have recognised high-level terraces 
in the Helena A'alley just to the east of the Ridge Hill area. These terraces 
fall into two series, one lying at about 4o0 feet, the other at about 2o0 
feet above sea level. The 250 feet series mav be represented in the Ridge 
Hill area by the Hidge Hill Shelf and the 450 feet series by the tiattened 
spur south fr(un Stathams (piari'y, but otherwise these terraces cannot be 
detected in this area. The llattened st)urs both in the Darlington area 
(with the exee])tion of the terrace on which the village of Darlington 
stands) and above Stathams (juarry are cored with epidiorite dykes. As 
has been noted above the epidiorites are less resistant to erosion than the 
granitic rocks — is it possible therefore that those flattened spurs or 
terraces are due to the differential erosion of Aveakly resistant epidiorite, 
more resistant gTanite and most resistant contact altered granite as indi- 
cated in text hgure 1, rather than to two periods of still-stand during 
the uplift of the Darling Platea-U? 
III. GEOLOGY. 
A. FIELD DISTHIBUTION' AND RELATIONSHIPS OF THE ROCKS. 
The diagonal joining the north-east and south-west corners divides 
the area conveniently into two parts. To the east and south of this line 
the I’ocks are those of the Pre-Cambrian granitic complex with associated 
