BY T. GRIFFITH TAYLOR. 339 



C. (Ratio silt/water) varies with the different rivers from 1 in 2000 to 1 in 

 ^00. The former value would double the period in years, and is perhaps 

 more nearly correct as the Po drains a glaciated country covered with debris. 



D, The silt in Lake George was supposed to occupy a wedge 10 miles long, 

 5 miles wide, with base 100 feet deep. (This is probably too small a bulk). 

 This gives a volume of 68,500 million cubic feet. Since a cubic foot of sand 

 weighs about 100 lbs., this represents a weight of 3,400 million tons as 

 deposited during a period of x years. 



As the area in question is yj^ ^^ that drained b}' the river 

 Po, and the rainfall and silt-carrying power less for Lake George, 



, , , 67,000,000 X 25 x 900 

 we may roughly put down IQQ x 30 x '^000 — ^^ amount 



deposited in Lake George in one year ( = 250,000 tons). 



XT Q^nn -ir ^ -ii u j -^ i • 3,400,000,000 

 Hence 3,400 million tons will be deposited in ^ '- 



years, or say roughly 14,000 years; a result which is quite as 

 near the truth as could be expected. No account has been taken 

 of the velocity-factor, which is very important in connection with 

 silt-carriage. If the velocity were lower than the mean velocity 

 of the Po, it would increase the period. If the rainfall, as is 

 probable, were heavier in prehistoric times in Australia, it would 

 decrease the period. However, one may perhaps be permitted to 

 set down this huge senkungsfeld as having taken place less tiian 

 twenty thousand years ago. 



vii. Summary. 



Lake George, the largest lake in New South Wales, occupies 

 an area of subsidence (senkungsfeld) bounded on the west by a 

 fault plane of about 400 feet drop. The fault is approximately 

 parallel to the strike of the Palaeozoic slates and phyllites. It 

 runs north and south for thirty miles, and constitutes the CuUarin 

 Range. The violent tectonic changes have entirely altered the 

 drainage-system of the district. The Molonglo flows through a 

 gorge it has cut in the CuUarin Range, and is clearly an antecedent 

 river. The feeders of Lake George once formed part of the Yass 

 River system. Their lower portions are buried under the silt of 

 Lake George, and they thus fall into the class of betrunked 



