^'^P'-'l Shephard, a J'isit to Great Lake, Tasmania. 71 



1916 J ' ' 



From the south and south-east the rise to this plateau is gradual, 

 and the road from Hobart via Bothvvell to a little over 3,300 feet 

 at Great Lake has no severe grade in the 80 miles traversed. 

 This plateau shows on the surface a mass of weathered diabase 

 or dolerite. with scattered patches of basalt. It presents a 

 singularly interesting appearance, broken up as it is by ridges 

 and low l^luffs, and in the northern portion rising into con- 

 siderable eminences. Geological research has brought evidence 

 to show that this huge sheet of diabase covering the plateau, 

 capping Mount Wellington in the south and Ben Lomond and 

 Moiint Barrow in the north-east, showing also at a lower 

 ele\-ation in Cataract Gorge, near Launceston. and Paradise 

 (iorge. at Orford, on the east coast, is what is known as a 

 " sill." The classical instance of a sill is the Great Whin Sill, 

 in North Yorkshire. This has been traced for 80 miles, and 

 covers an area of about i.ooo square miles. If the diabase of 

 Tasmania can be regarded as one sill, then it is a worthy rival 

 of the Great Whin Sill. This diabase is a rock not dissimilar 

 (on cursory examination) to the well-known bluestone of Vic- 

 toria, but does not show vesicular structure, being compact. 

 It is composed of augite, felspar, labradorite. and magnetite, 

 and sections from specimens from three localities so wide apart 

 as Great Lake. Cataract Gorge, and Paradise Gorge showed 

 no difference in the mineral constituents, but only a difference 

 in the size of the crystals. Regarding this mass of diabase as 

 a sill or a series of sills, the extent of the denudation required 

 to remove the material originally covering it and that inter- 

 vening between the main mass and the outliers is enormous, 

 and is another of the many instances justifying the geologist 

 in his. extreme liberality with time. On this plateau, towards 

 its northern end. Great Lake is situated. 



An interesting trip can be made by road to Tasman's 

 Peninsula. The Derwent is crossed by ferry to Bellerive, and 

 from thence over a causeway crossing an inlet known as Pitt- 

 water. i)assing the town of Sore 11. The first isthmus is reached 

 at Dunally ; this is narrow, and, crossing it, Forestier's Peninsula 

 is entered. The scenery from here onward is charming ; forest- 

 clad hills alternate with seascapes with kaleidoscopic rapidity. 

 Eaglehawk Neck is a connection between Forestier's and 

 Tasman's Peninsulas. Historically famous for reasons a lover 

 of Nature may disregard, it is extremely attractive for the 

 physiographical features it ■|)resents. The isthmus is only 

 some sixty yards across. On the east, ocean surges roll into 

 Pirates' Bay : to the west, the placid waters of Norfolk Bay ; 

 to the south and north are seen steep tree-clad hills. Marine 

 denudation here exhibits some unique results. At the northern 

 end of Pirates' Bay, the sea, eating into the yielding Pernio- 



