RIVERS OP NEW SOUTH WALES. 397 



With a catchment area of 6,4/35 square miles, a mean rainfall 

 of thirty-four and two-third inches and a length of two hundred 

 and forty-five miles, the Tiber bears a fairly close analogy 

 to some of our coastal rivers, and may, in the absence 

 of a better instance for comparison, be taken to illustrate 

 the probable amount of work done by the Murrumbidgee at 

 Wagga in transporting silt. It has been ascertained that the 

 quantity of silt carried by the Tiber is ^jVs ^7 weight of the 

 water discharged. If we assume that the corresponding pro- 

 portion in the case of the Murrumbidgee is yj'fj^, and that 

 the mean discharge for the year is 2,000 cubic feet per 

 second, we find that the quantity of silt carried past Wagga 

 on an average every twenty -four hours is 4,018 tons. This 

 in a year would amount to about one million cubic yards, 

 or slightly less than a depth of ,^n^ of a foot over the catchment 

 area. In other words, on this hypothesis the catchment area of 

 the Murrumbidgee above Wagga is wearing down at the rate of 

 a foot in twelve thousand years. The proportion of silt assumed 

 is intermediate to the proportions ascertained in the cases of the 

 Ganges and the Mississippi, that of the former being ^^f^ and 

 that of the latter j;^. In all probability the rate of denudation 

 on the catchment area of the Murrumbidgee is considerably in 

 excess of that here obtained for we know that on the basin of the 

 Rhone the loss by denudation is at the rate of a foot in one 

 thousand five hundred and twenty-eight years ; in that of the 

 Upper Ganges, a foot in eight hundred and twenty-three years ; 

 and in that of the Po, a foot in seven hundred and twenty -nine 

 years. 



It is necessary to mention here that while the process of denu- 

 dation which has resulted in the formation of great alluAaal 

 deposits along and near the courses of our rivers is still in pro- 

 gress, there is much reason to believe that at some former periods, 

 the rate at which this action took place was greatly in excess of 

 what it is under present conditions. In connection with this it 

 has long been well-known to geologists that the conditions of 

 climate in the Northern Hemisphere have ditiered widely in 

 different geological periods, and that at a comparatively recent 

 period the British Islands, and nearly the whole of continental 

 Europe, had a climate such as is not now found south of Lapland. 

 The explanation of this fact which receives most general accep- 

 tance is that which was first propounded by Dr. J. Croll, and 

 brought into prominent notice in his great work " Climate and 

 Time in their Geological Relations." It would be out of place 

 here to do more than mention that in the work referred to. Dr. 

 Croll has shown that the remarkable changes of climate which 

 different parts of the earth have undergone at different periods, 

 can be accounted for by the changes which are known to have 



