C. C. Brittlebank—Rate of Erosion of River Valleys. 321 



ancient Werribee, but the latter was blotted out of existence by the 

 flows of basalt. The Lorderderg, being guarded by high Silurian 

 ranges on the south over which the lava did not rise, continued in 

 the same channel as it now occupies. I will therefore confine my 

 remarks to the Werribee and tributary creeks. 



The formations through which tlie Werribee has cut are : Lower 

 Silurian (Ordovician), granite, Glacial Series, fresh- water 'Miocene' 

 gravels, and basalts, both older and newer. The newer basalt has 

 a,t one time formed an extensive plateau from Mt. Blackwood, the 

 Lerderderg Ranges, and Ballan on the W. and N.W. to the Brisbane 

 Kange and the plains on the S. and S.E. The surface slope is to the 

 S.E., and is broken here and thei'e by ridges and hills of the various 

 older formations which stand at a higher level. In making my 

 observations I have used the present surface of the basaltic plateau 

 as a bench mark. Careful measurements have been taken from the 

 surface to the river bed at all places where erosive action is being 

 checked. 



The different rocks through which the rivers have cut their 

 valleys greatlj^ influence the amount and direction of erosion and 

 denudation. I find that the resistance offered by the various geological 

 formations to the erosive power of river action is in the following 

 order : Basalt, Silurian Granite, Glacial Series, Miocene ferruginous 

 gravels. Jointing, dip, strike, dykes, and bands of hard rock have 

 considerable influence in aiding or retarding the river action. 

 Fallen rock masses, especially huge blocks, check erosion to a con- 

 siderable extent. A band of hard rock crossing the river valley 

 acts in two ways, first by contracting the channel and second by 

 checking vertical erosion below its own level on the up side. 

 Numbers of these bands cross the stream, and many of my observations 

 have been taken from them. Probably the best position for observing 

 the wearing of any river channel is at the outer angle of a sharp 

 bend. At such places the bed is almost always free from boulders, 

 gravel and sand. Moreover, by the constant undermining action 

 of the river an almost vertical cliff is formed on the outer bend. 

 The greatest advantage, however, lies in the fact that the upper 

 part of the river rarely moves from side to side, but cuts a fairly 

 straight course through the country. 



I had expected that the erosion of the river valleys would be 

 carried on at a comparatively rapid rate, but after many observations 

 along their courses I find the amount removed in a period of five 

 years is very small, so small in fact that had the amount not been 

 checked at numerous points I should hesitate to place on record 

 the results, but having undertaken the observations with this object 

 in view I give the average obtained at many separate stations on 

 the Werribee and tributary creeks. 



Basalt 0'02 inches iu 5 )ears = l iuch in '2')0 years. 



Silurian 0-03 ,, ,, = ,, 166§ ,, 



Granite 0-04 ,, ,, = ,, 125 ,, 



Glacial 0-05 ,, ,, = ,, 100 ,, 



Having ascertained the amount of erosion over a given period, it 



DECADE IV. VOL. VII. — NO. VU. -1 



