34 NOTES ON THE GEOLOGY OF CRADLE MOUNTAIN DISTRICT, 



filled with quartz, carbonates, and haematite. There is 

 a remarkable absence of magnetite and of glass. 



The Pleistocene and Kecent deposits will be best con- 

 sidered with the general physiography, which we now pro- 

 ceed to discuss. 



The writer is indebted to Mr. Twelvetrees's report on 

 the adjacent Middlesex district for an account of the 

 general relation of the physiographic features to the re^ 

 gional topography of Tasmania. He states, "The entire 

 area isi an elevated plain or tableland, dissected by stupen- 

 dous gorges, and diversified by residual mountain ranges." 

 He indicates that the tableland is separated by faults 

 near Mt. Roland and Bell Mt. from the lower plateau 

 near Sheffield and Wilmot, and lies at an elevation 

 of 2, 200-2, 600ft. in the neighbourhood of Middlesex. (H) 

 It rises gradually to the south-west, and around Cradle 

 Mt. it lies about 4,000ft. above sea level. The plateau 

 lias here cut across the uneven surface of contact of the 

 Pre-cambrian and Permo-carboniferous rocks, so that the 

 surface of the plateau consists of irregular areas of the 

 two formations. The more siliceous Pre-cambrian rocks 

 rise in small residuals, but the three dolerite mountains 

 form the greatest monadnocks. The plateau is trenched 

 by the great gorges of the Forth River and its tributaries, 

 to the east of Cradle Mountain, and by the gorge of the 

 Fury on the west. The effects of the Pleistocene glacia- 

 tion arei everywhere visible, and to these we will devote 

 special attention. 



Six periods may be recognised in the development of 

 the present topography. In the first, possibly early Ter- 

 tiary period, the dolerite-sills were laid bare by erosion, 

 and a roughly horizontal surface of erosion or peneplain 

 was produced in the dolerite. An uplift followed of more 

 than ?- thousand feet, and the present peneplain-surface 

 was cut out of the older level, fragments of which remain 

 as residuals, such as Cradle Mt. and Barn Bluff. A 

 mature system of valleys was originated between these, 

 and, in particular, the course of the Forth River was out- 

 lined. The third period was one of oscillatory uplift, accom- 

 panied by gentle tilting. The numerous immense gorges of 

 the Forth and Pieman River systems were produced by 

 revival of the ancient matvired valleys. Tributary gorges 

 such as Hanson's, Rodway's, and the Fury cut themselves 

 right back to the foot of the residual mountains, while 

 others, such as Smith's Creek and the Dove River, and 

 Pencil Pine Creek, were considerably deepened in their 



(11) Bibliography No. 42. 



