246 ^^'^ ANTIQUITY OF MAN IN TASMANIA. 



5. THE CAUSES THAT PRODUCED THE 

 FORMATION OF BASS STRAIT. 



The successive pictures as revealed to us of the sub- 

 marine physiography of Bass Strait, should the sea level 

 gradually recede from o to 45 or 50 fatnoms, point con- 

 clusively to the agencies to which these features are due. 

 The submarine features of Bass Strait are unquestion- 

 ably not of primary origin, that is to say, they did not 

 exist before the submergence of the isthmus. The features 

 of present-day Bass Strait already indicate clearly 

 enough that its origin must be due to dynamic agencies. 

 In fact, Bass Strait can be considered almost as a model 

 of a basin of subsidence. Segments of the earth's crust 

 commenced to subside, slowly at first, more rapidly 

 afterwards, the force rapidly gaining in energy till the 

 maximum was reached, and then slowly decreasing 

 again. 



All geological observations tend to prove that an 

 area of subsidence is accompanied by volcanic outbursts; 

 either within the area or along its edges. Evidence of 

 an intensive volcanic action is common enough along 

 the coasts of Bass Strait. It would go too far to enume- 

 rate all the occurrences, but the submarine features, 

 together with its present outline, and the geological 

 evidence of the coast, prove conclusively that Bass 

 Strait was formed by the subsidence of portions of the 

 earth's crust, accompanied by volcanic outbursts. 



If this be so, the inland lake as depicted, PI. I., Fig. 

 8, probably never 'existed ; that is to say, when the first 

 subsidences commenced, ashes were thrown up, craters 

 were formed emitting lava streams, but as the subsidence 

 continued a large bay was formed, even ^before any in- 

 land lake could be established. 



. If we admit that the present submarine features of 

 Bass Strait are not primary — that is to say that they 

 were formed only towards the end of the period when 

 Tasmania still formed part of the mainland, we must 

 presume a period when they did not exist. As working 

 backwards has not helped us very much, it will be better 

 to start from a certain datum level, so to speak, and to 



