Geology of NillaJtcootie. 299 



The northern boundary of the reservoir would consist of a 

 concrete wall running from the western granite margin across 

 the riv-er bed to the granite at the other side* of the bed, and 

 then continued by an embankment resting on portion of the 

 conglomerate beds unless it were found necessary to run the 

 concrete wall right across the northern end of the reservoir. 

 If, as seems probable, this conglomerate is of glacial origin, 

 then there is certainly no necessity to sink through these beds 

 in order to let the foundations of the cnncrete into bed-rock, as 

 the conglomerate itself would possess quite as good holding 

 powers as the undcTlying granite and silurian. Should the con- 

 glomerate on further investigation be proved to be of glacial 

 origin then the loss by soakage through it might be taken as 

 being nil, and there could be no doubt about the suitability 

 of the area for a wear site. 



On the other hand there is the possibility of the conglomerate 

 turning out to be due to river action, and then there would be 

 the chanc'e of a considerable loss by soakage unless the concrete 

 weir was let into the underlying rock. Further, there would 

 be no comparison between the holding strength of such a river 

 deposit, and a conglomerate ol glacial origin. The sinking of a 

 single shaft through these beds should settle definitely their 

 origin and give a sure basis on which to estimate the probable 

 cost of the weir. 



The catchment area requires very little comment. The whole 

 area is practically surrounded by well-wooded hills, and as the 

 structure of the country is similar to that of the reservoir area 

 there would be little loss of water, because the water which did 

 soak into the ground on the hill sides could find no dutlet ex- 

 cept into the reservoir. Shut in as it is by hills the loss of 

 water from the reservoir by evaporation would be small. The 

 standard of purity of the water should be high, especially that 

 derived from Back Creek and Sandy Creek, flowing as they do 

 mainly over granitic rocks. The water which comes down the 

 main river in flood time carries a considerable amount of red 

 sediment in suspension derived from the carboniferous rocks, 

 but the water contains veiy little dissolved matter. The area 

 to be served is part of the Miu'ray Plains, which have apparently 

 been formed by the building up of the whole engrafted Murray 



