BY A. N. LEWIS, M.C. 17 



AN OUTLINE SKETCH OF THE GEOLOGY OF THE 



NATIONAL PARK OF TASMANIA. 



The National Park of Tasmania is located about 50 

 miles from Hobart, among the Mt. Field ranges, on the 

 northern slope of the Russell Falls River, a tributary of 

 the Derwent. It comprises some 38,500 acres of wild moun- 

 tain tops and dense forests, and contains the most varied 

 scenery within a day's trip of Hobart. The Park was 

 originally set aside in an endeavour to preserve some native 

 fauna and a little of the romantic virgin bush from the 

 depredations of a misguided civilisation, but with the open- 

 ing up of the area it has been discovered that there are 

 contained within its boundaries geological features of con- 

 siderable interest. 



Topographically the Park is a portion of the very 

 much dissected Central Plateau of Tasmania, itself cut 

 off from the remainder of the plateau, and isolated by 

 the valleys of the Derwent and the Russell Falls Rivers. 

 The smaller plateau so formed is itself a dissected table- 

 land. This tableland consists of diabase that intruded into 

 the older strata of most of Tasmania during the Cretaceous 

 period, and the general topography conforms to the outline 

 of this diabase. Probably the area was raised to its pre- 

 sent height in one uplift by this diabase, contemporaneously 

 with the elevation of the Mt. Wellington Range to the south 

 and the Central Plateau to the north. Any overlying rocks 

 carried up by the intrusion have since been removed by 

 erosion. 



In the past the Mt. Field Range has been considered 

 a portion of the Mt. Wellington Range. This appears to 

 me to be stretching the term mountain range too far. The 

 diabase is undoubtedly of the same age, but the Mt. Field 

 Range is separated from the Mt. Wellington Range by the 

 Tyenna Valley. This valley is not entirely waterworn. 

 The West Coast rocks extend into it at an altitude of 

 less than 1,000 feet, and Ordovician limestones circle round 

 from the Florentine Valley to the Junee. Above these, large 

 beds of Permo-Carboniferous and Trias-Jura sediments 

 bound the western face of Mt. Field West, and the entire 

 northern slope of the Tyenna Valley, and there is no surface 

 connection between the diabase of Tyenna Peak and that of 

 B 



