Surface Tension and Denudation. 57 



evaporated. It is material which was in mechanical suspension, 

 being carried down and acting in its turn as a file to wear away 

 still more of the loosely cemented hill-side. 



Even where there is water falling over into the canyon, .some 

 of it always trickles down adhering to the surface. It then 

 quietly trickles out any softer band. Surface tension, therefore, 

 greatly helps the splash in the wearing under by a waterfall. 



In canyon formation, especially in the advance of the main 

 canyon upstream and in the lateral widening of the canyon, 

 surface tension is seen to be an agent of the first importance by 

 enabling the water to adhere to the rock. This water thus runs 

 over the softer bauds, otherwise protected from water action. 

 These are quickly worn out and removed. The harder beds 

 being unsupported, now break off and fall in, and so the canyons 

 grows. Thus the undercutting can in many cases be said to be 

 directly due to tlie eflfect of the surface film of water. 



A recent visit to the Coburg bad lands, east of Pentridge, 

 showed many canyons, varying in depth up to nearly 20 feet. 

 In not one single case could any trace be discovered of a splash 

 at the bottom of the fall at the head of any of the several canyons. 



As a teacher, difficulties have been experienced in leading a 

 class to understand how on a slope, say N. and S., a lateral 

 tributary might cut across this slope from E. to W., and the 

 streams flowing on it be captured and diverted. There is some 

 reason for believing some of the present rivers have so cut across 

 the old southerly flowing rivers of Central Victoria. 



To illustrate this an excursion was undertaken to Kilby 

 Lagoon, near Kew. Here on the hillside is a canyon with 

 several lateral tributaries. These carry but little water — ^just a 

 trickle after rain in fact — yet the softer material has been 

 quickly trickled down until now a lateral tributary, flowing 

 entirely underground (for the surface crust, supported by the 

 thick mat of grass roots, has not yet fallen in), drains much of 

 that hillside, though it is sloping north, out to the west. 



This wearing-out has been accomplished in a very short space 

 of time, for the hillside has been ploughed, as the old furrows 

 plainly show. 



Thus surface tension, acting indirectly, is seen to be an agent 

 in river capture by quietly but surely working a .side tributary 



