35 
One of us (M.A.) considers that the larger brooks issuing from 
the Darling Range Escarpment; in the Swan River District show 
distinctly younger valley profiles in the lower parts of their courses 
than further upstream. As examples Jane Brook, and Narrogin 
Brook (Armadale), are quoted. This feature is connected with the 
whole period of rejuvenation. it is necessary, however, to bring 
to notice the factors which may assist or retard corrasion in these 
streams. 
( orrasion may be* assisted by the jointing and faulting of 
the crystalline plateau. Jointing is difficult to trace, but might, 
he expected to have been recorded by a yielding formation such 
as the Armadale shales. The joint systems of this formation 
have been plotted, and are found to belong to two series, each 
having two sets of joints at right angles, showing in all twelve 
separate directions. 
First series — 
Bet a . . N.N.E. by N. to S.S.W. by S. 
Set b . . E.S.PL by E. to W.N.W. by W. 
Second series— 
Set a . . E.N.E. by N. to W,S.\V. by S. 
Set b .. NA\ r . by 1ST. to S.E. by S. 
The former series is the stronger, but none of these directions seem 
to be prominently marked in the drainage of the plateau. Fault- 
ing is more easily recognised. From an examination of the Arma- 
dale District, we conclude that the Narrogin Brook and its northern 
tributary follow a line of fault behind the foothill zone (15), hav- 
ing found a displacement of laterite level in this zone, amounting 
to about 200 feet, on the south side of the (binning’ Valley. Later- 
ite displacement, of this amount is a clear indication of recent 
faulting (12). A similar displacement has also been observer! 
between Jane Brook and its northern tributary well within the 
Range, and the course of Jane Brook behind (lie foothills zone re- 
sembles that of Narrogin Brook. It is suggested that the foothill 
zone is always ■ a product of sfej > faulting. 
Factors which may assist in the development of local maturity 
are of the nature of temporary base-levels. The only competent 
obstacles are the basic dykes. From a careful study of their out- 
crop's we believe that the basic dykes are more easily weathered , but 
less easily eroded, than the granites of the district. The difference 
in both cases is very slight. Where a dyke crosses a hill at right 
angles to the contours it weathers out, leaving a col. Lower down 
the hillside the same dyke forms a ridge, for here erosion is more 
rapid than weathering. Similarly, where a dyke is parallel to or 
inclined to the contours at a low angle, the outcrop forms a dis- 
tinct ledge on the hillside. So far, however, we have not seen a 
dyke which has become an obstacle or temporary base-level for a 
stream. 
