162 ./. T. JiUson: 



Bundoora, emptying itself into the Yarra, where that creek now 

 •enters. The lateral overflow of the basalt changed this. Tlie 

 stream naturally kept between the basalt and the siliirian, and 

 so entered the younger valley. Once there it retained its course, 

 keeping the thin basaltic strip to the west, and excavating 

 much more deeply than the level of the basalt. Hence the 

 diversion of the upper Plenty waters through the Greens- 

 borough country to the Yarra to the west of Templestowe. 



The fall of the narrow strip of basalt in the old independent 

 stream has already been noticed. It is interesting to record 

 the fall of the present river in various parts. Between Greens- 

 borough (river bed about 100 feet above sea-level) and the 

 southern tongue of basalt already referred to (river bed about 

 200 feet above sea-level), a distance of over 3 miles, the fall 

 would be about 30 feet per mile. Between this southern tongue 

 and South Yan Yean (river bed about 500 feet above sea-level) 

 a, distance of about 5 miles, the fall w^ould average about 60 feet 

 per mile.l If a mile to the south of South Yan Yean be left out 

 of the calculation, the fall would be greater, as the stream in 

 this mile runs entirely through basalt and its valley is shallow 

 (not more than 30 feet deep). When the basalt has been cut 

 through and the softer silurian rocks met, erosion acts much 

 more rapidly. About South Yan Yean the river continues in a 

 very shallow valley, whose fall would approximate to that of 

 the country through which it runs. Tlius betw^een South Yan 

 Yean (532 feet) and Whittlesea (637 feet) a distance of about 

 6 miles, the difference in height is 105 feet, giving an average 

 per mile of \7h feet. 



Tlie figures quoted bring out two points very clearly. Firstly, 

 the gentle grade of the valley, cut in a pre-existing one, 

 as com:pared with the steep grade (once the basalt is cut 

 through), w^here the river had to cut entirely afresh. This is 

 what would be expected. Secondly, the Avonderful power of 

 resistance to denudation of the basalt, compared with, the silu- 

 rian sediments (although these in places are fairly hard rocks). 

 Where the stream has not cut through the basalt into the under- 

 lying rocks, the valley is shallow and insignificant, but where 

 such cutting has been done a deej) gorge exists. 



1 Kxccpt the heitjlits of railway stations and tliose mentioned on the Quarter Sheets 

 those ffiven in this part of tiic pajter Iiave lieen determined by aneroid. 



