•62 GEOLOGY OF TASMANIA. 



characterises the Silurian strata throughout the Island. 

 Interbedded with the sedimentary beds at Zeehan are sheets 

 of Silurian basalt (melaphyre), known locally as " white 

 rock." This is often tuffaceous and vesicular. In the 

 Oonah and Montana mines, it may be seen in the form of 

 contemporaneous sheets. 



Of about the same age are slates, sandstones, and lime- 

 stones in the Bell Mount district, between the Forth and 

 Wilmot rivers. The sandstones there and at Mount Claude 

 contain abundant casts of fucoid stems; fenestella, trilo- 

 bites, and rhynchonella also occur at Bell Mount and the 

 Five-mile Rise. Clay slates, with calymene, orthis, car- 

 diola, in the Eldon Valley, are referred to the Upper 

 Silurian. 



Associated with the rocks of the system in the N. and 

 W. is an extensive development of serpentine, the altered 

 form of gabbro and its appendages, peridotite and pyroxe- 

 nite. Dykes of it cross the Silurian strata on the road 

 between Waratah and the Whyte River, and the rock 

 underlies metamorphosed sandstones at the Heazlewood. 

 A great variety of gabbros and pyroxenites may be seen 

 along this road. Nickel Hill, at the Sixteen-mile, is a mass 

 of serpentine rock, containing nickel ores, and Bald Hill, 

 immediately to the v/est, is likewise serpentine as far as the 

 Nineteen-mile, where it impinges against Silurian slates. 

 A pyroxenite dyke in Silurian strata carries the silver-lead 

 lode at the Magnet Mine. Gabbro, pyroxenite and ser- 

 pentine occur in the Dlindas district, and reappear west of 

 the Comstock, and again at Trial Harbour. In the Valley 

 of the Forth, and at Anderson's Creek, west of Beacons- 

 iield, further areas of serpentine are exposed, and at the 

 latter place the rock is often asbestiform, and is mined for 

 asbestos. It is difficult to assign a precise age to our 

 gabbros and serpentine. They have been thought to be 

 pre-Silurian ; but the Heazlewood intrusions suggest the 

 close of the Silurian as a possible date. 



Very important rocks are the quartz-porphyries, or fel- 

 sites, which form the backbone of the West Coast Range. 

 These are the geographical axes of Mounts Darwin, Jukea, 

 Huxley, Tyndal. and continue northwards through Mount 

 Murchison, and on the east side of Mount Farrell. They 

 are the home of copper ores, and enclose characteristic de- 

 posits of hematite and magnetite. Chloritic copper-bearing 

 schists, some of them probably schistose porphyries, flank 

 them, and are enclosed in them. On the whole, the quartz- 

 porphyry is massive, but it occurs also laminated. It w£i3 

 probably intrusive, but this can only be decided after 



