510 J. T. Jutson: 



by the island becoming attached to the ".' mainland " — that is, 

 to one side of the valley. Actual examples of growing and of 

 ■completed attachment may be seen in the Yarra above War- 

 randyte. If the island happens to be fairly close to one side 

 of the stream, drift wood soon collects in quantity in the 

 narrow channel. The cun-ent becomes checked, silt is de- 

 posited, and the island gradually becomes tied to the main- 

 land, forming a strip of alluvial land with dense vegetation. 

 A certain portion of the river is therefore diverted, and natu- 

 rally, other conditions being suitable, the stream tends to cut 

 into the opposite bank and form a curve. This curve may in- 

 fluence the whole course of the river down-stream. 



The writer is not aware that the connection here noted be- 

 tween rapids and small islands in rivers, has been hitherto 

 recorded. 



Differential Pot-Hole Erosion. 



In most text books of geology and physiography, the forma- 

 tion of pot-holes in the beds of streams is described, and their 

 effect in the general erosion of the valley noticed. The writer 

 has not, however, seen it distinctly stated^ that this mode of 

 erosion possesses a selective action, although this is perhaps 

 implied in the general statement that hard rocks resist the 

 denuding agents much more eft"ectually than soft ones. As 

 an actual illustration of such action has been observed, the 

 following remarks may be of some general as well as of local 

 interest. 



TTie example referred to occurs in a small outcrop in the 

 bed of the Mullum Mullum or Deep Creek, at a point imme- 

 diately to the north of the Deep Creek Road, which meets the 

 stream just after its pronounced swing round from the south- 

 west to the north-west, near Ringwood. The rocks consist 

 mainly of shales of medium toughness in beds from three or 

 four inches to about three feet in thickness. Interbedded with 

 these are bands, from one inch to three inches thick, of hard 

 micaceous sandstone. The strike of the rocks is approximately 



1 The nearest ai)proach is in K. S. Tarr's " New Pliysioal Geography" (190f)), which 

 contains a photograph of cascades falling (presumably over hard rocks), and excavating 

 pot-holes in shale*. The two cases however are not ([uite parallel. 



