T. STEPHENS, M.A., F.G.S. 



■7! 



The origin of the specimens of agate, carnelian. and 

 various forms of chalcedony, which are often found in 

 gravels of the Derwent basin, or ploughed up in 

 basaltic soils, has always been something of a mystery,. 

 but the occurrence of these opals in situ points to our 

 Tertiary basalt as one of the sources from which they 

 have been derived. 



The basaltic country which has been described abuts 

 against a rather lofty rise of deep bedded gravels with 

 quartzite boulders up to about eight or ten inches in 

 diameter. The summit of the hill is, by aneroid, about 

 440 feet above sea level. These gravel beds show no. 

 sign of local glaciation, but may be moraine matter 

 brought down by post-glacial erosion. Before any de- 

 finite conclusion can be formed respecting the history 

 of these gravels and boulders, it will be necessary to 

 investigate the history of similar deposits in other parts 

 of the Derwent basin. On the slopes of the eroded 

 sandstones between Glenora and Hamilton, some 200 

 feet above the present river level, are lines of travelled 

 shingle and waterworn boulders, and a similar deposit 

 lies high up on the ridge between Hamilton and Upper 

 Broadmarsh. These may be regarded as the remains 

 of terraces on the margin of ancient lakes long since 

 drained by erosion of the river bed. But it is to be 

 noted that none of these deposits are of local origin. 

 All the material consists of quartzites, schists, and in- 

 durated sandstones, which have come from the far dis- 

 tant Western country, and their distribution is sugges- 

 tive of some form of glacial transport. 



Approaching- Fenton Forest the hne passes through 

 a small rise of sand and fine gravel. On the right are 

 the hop grounds and paddocks occupying what was the 

 bed of one of the numerous lakes of the Derwent Valley 

 before the river cut its way through the barrier of basalt 

 near Macquarie Plains. So far there was. no formidable 

 obstacle to the construction of the railway; but from 

 near the Forest gate the cuttings for a distance of nearly 

 two miles are through massive diabase of an unsually 

 refractory character. At two and a half miles from 

 Glenora was the maximum difficulty of the whole line. 

 The diabase of Eastern Tasmania is notoriously one of 

 the hardest and toughest of rocks, but here there was 



