17' 



NOTES ON THE HOBART TOWN STORAGE 



RESERVOIR. 



By T. Stephens, Esq., F.G.S. 



[Read Septemler llth, 1877.] 



The question of the water supply of Hobart Town is one of 

 annually increasing importance. The supply itself has been largely 

 increased, but is, and must continue to be insufficient, so long as no 

 provision is made for securing a reserve which may be utilised 

 when the direct service from Mount Wellington is reduced by a dry 

 season, or by other causes. Until a further provision is also made 

 for filtering the water before it enters the town, it must continue to 

 bring with it the impurities with which it necessarily becomes 

 charged during the passage through open channels or imperfectly 

 covered flumes. Attention must soon be directed to the now use- 

 less storage reservoir, on which so large a sum has been expended 

 to no useful purpose, and it may be well to inquire into the causes 

 of failure, and consider whether there is any hope of its ever being 

 made even partially available for the objects for which it was de- 



It is almost superfluous to remark that the whole of the area 

 lying between the Derwent and Mount Wellington has been sub- 

 jected to very great geological disturbance. Any one passing up 

 the Huon road may see at a glance that the sedimentary rocks have 

 been greatly dislocated by the intrusion of igneous rocks in vast 

 masses and dykes, producing a variety of disturbances among 

 the sandstones of the locality. At the toll-gate, however, there is 

 a more important displacement which does not appear to have been 

 caused by merely local intrusions. Here a vast mass of the mud- 

 stone, a member of the Upper Palseozoic Series of the southern 

 rocks, has been vertically elevated, together with the originally 

 overlying sandstones, some of the remains of which may be seen on 

 the opposite side of the Sandy Bay Rivulet. The relations of the 

 mudstone to the sandstone formation which abuts against it are, 

 at this point, somewhat obscure ; but on both sides there 

 are unmistakeable indications of an extensive fault, which 

 appears to cross the Hobart Town Rivulet, not far from the 

 Cascades Brewery, and I have no doubt that it traverses the valley 

 occupied by the storage reservoir almost, if not quite, on the very 

 site of the dam. Here then is a very simple explanation of the 

 cause of the leakage which I have been told gave a good deal 

 of trouble soon after the embankment was first made, and which 

 has been, I believe, attributed to landslips. This, however, is not a 

 case of simple landslips, though they will always occur under like 

 conditions. Where an extensive fault of this description has been 

 occasioned by the violent disruption of a vast mass of variously 

 compacted rocks there is necessarily a fissure of unknown .depth 

 along the line of fracture, and the rocks on both sides being more 

 or less shattered by the grinding process to which they have been 

 subjected, will surely slide and settle down whenever they have 

 been undermined by natural and other causes, as in the present 

 instance. Another great fault crosses the valley close to the upper 



