Failure of the Yan Yean Reservoir* 



141 



Mr. Dempsey, himself, entertains the same views on this 

 subject as I have now expressed^ and thus the favourite 

 authority of the Committee would be the first to detect their 

 illogical reasoning, and he would be especially astonished, 

 that on such reasoning they relied for justification of their 

 unwarrantable adoption of his evaporation tables, to determine 

 the very important question of the watershed of the Plenty 

 basin, with a totally different temperature from that of 

 England. 



I have said that about one-fourth of the drainao:e area of 

 the Plenty is clay slate, and I may add the whole of that of 

 the reseiwoir, and it is the opinion of Mr. Blandowski, who 

 has made a careful geological survey of the whole colony, 

 that the clay slate formation is entirely destitute of rivers. 



The rain water quickly disappears through the surface soil, 

 which is composed of the detritus of the slate, and is lost 

 chiefly by evaporation from the surface, which becomes 

 intensely heated by the solar rays, and partly by absorption 

 through the seams and fissures of the strata, to re-appear as 

 springs, at some lower level, either in the ocean or in the beds 

 of rivers ; or, as frequently happens, in the waterholes of the 

 dry creeks and watercourses which have no other permanent 

 supply in the summer months ; and this circumstance has led 

 some to entertain very false notions with regard to the 

 evaporation from the surface of water in this colony. Find- 

 ing that their waterholes, on which they depend for their 

 domestic consumption, suffer little diminution in the heat of 

 summer, they conclude that the evaporation is very trifling. 



Artificial waterholes, sunk in any locality, where the close 

 structure of the underlying rocks prevents the escape of the 

 surface water, will lead to the same erroneous conclusions. 



With scarcely any exception, the slate strata are vertical, 

 running north and south. Hence, they present the most 

 favourable condition for absorption ; and it is very common 

 to find rivers originating in the granite formation, gradually 

 losing themselves in the districts of the slate formation. 



I have said that the Committee have calculated the amount 

 of watershed, in accordance with Mr. Dempsey's tables. I 

 do not say that their calculations are based on these tables ; 

 but, to use their own expression, they check their calculations 

 with them. By a method which has never before been applied 

 to determine the watershed of any other country, they find 

 that 57'6 per cent, of the rain in the Plenty basin is evapor- 

 ated, and 42*4 per cent, goes to the river, and then, in order 



