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