The Geology of the Darling Scarp at Ridge Hill. 
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(liorite dykes. The divide between these two streams, at 15 chains south 
of Kidge Hill siding', is occupied by an epidiorite dyke. 
The role that the epidiorite dykes play in the development of these 
minor topographic features of the Darling Range is of interest. In most 
places in the Darling Scarp area the granitic rocks are nioi’e resistant 
to erosion than the basic dykes which are generally represented by shallow 
depressions (text tig. lA) or fattened areas on otherwise uniform slopes 
Text fig. 1. — Relative resif-lance to erosion of granitic rocTcs and basic dylccs in 
the Darling Ranges: 
A. Diagrammatic sketch plan of granite ridge at Armadale (not to scale) 
showing that dolerite dykes occupy the saddles in granite ridges. 
B. Diagrammatic sketch section (not to scale) of upper part of the Darling 
Soarp at Kidge Hill showing minor fiattened lieiiches wliicdi are under- 
lain by epidiorite dykes. 
C. Diagrammatic sketch section (not to sctile) of geological structure of 
ridges in the Toodyay District showing that altliougU ridges are coi'cd 
by dolerite dykes there is a shallow central depression. 
(dranitic rocks arc indicated by crosses, basic dykes by arrow heads and 
laterite by circles.) 
(text fig. IH). Similarly at Toodyay a most noticeable feature is that 
the main ridges in the granite gneiss areas have a central core of dolerite 
but the crest of such ridges has a shallow central depression over the 
dolerite dyke (text fig. 1C). It is evident, therefoi’e, that in the Darling 
