BY FRITZ NOETLING, M.A., Ph.D.:, ETC.- 



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hind the walls of lava, those fresh water lakes were 

 formed whose deposits we now find everywhere in 

 Northern Tasmania. 



It is difficult to say how long this period of volcanic 

 activity has lasted. It is possible that everything was 

 over with one great eruption ; but it also seems as if 

 there have been several successive eruptions. We have 

 seen above that the greatest changes took place between 

 the 35 and 45-fathom line. If we assume that the greatest 

 volcanic activity w^as finished at the period represented 

 by the 35-fathom line, the volcanic period would have 

 lasted about 3,000 years. 



It is pretty certain that during the volcanic period, 

 and probably also for some time afterwards, the subsi- 

 dence of the surface continued : it is, however, also cer- 

 tain that when the sea had reached the 35-fathom level 

 the subsided area was filled with water. The craters (i), 

 consisting mostly of ashes and loose material, were 

 washed away, and a great bay or almost inland sea v/as 

 formed, which communicated in the west by a narrow 

 strait with the open ocean. The road for the migration 

 of the Adelaidean fauna was open, but it could not move 

 beyond I47deg. long., because an isthmus still intervened 

 here. 



A further rise of the sea level for 30 feet, correspond- 

 ing to a period of 1,000 years, considerably reduced the 

 width of the eastern land bridge, and we see that now 

 only a very narrow isthmus connects Tasmania with 

 Australia. 



Another rise of 30 feet, corresponding to a period of 

 1,000 years, is sufficient to separate Tasmania perma- 

 nently from the mainland. 



The further changes do not interest us much, except 

 that the last period, including the stages from the 25- 

 fathom Hne to the present level must have lasted 5,000 

 years if the sea rose at the rate of 3 feet per 100 years. 



The comparatively short period of 5,000 years fully 

 explains w4iy the Adelaidean and Peronian faunas are 

 still so sharply divided. The time since the opening of 



(i) It is very probable that a considerable quantity of the 

 material which formed the craters was redeposited by the water, 

 thus levelling up again the depressions. 



