BY FRITZ NOBTLING, M.A. PH.D. 97 



^1913. 



Mr. R. M. Johnston has already noticed that the leaf 

 bed series as a. whole, wherever they occur, are not found 

 higher than about 40 feet a,bove the present sea level, and 

 from this psculiai^ deposition he concludes that, excepting 

 for the south-easterly dip, the leaf bed series is practically 

 in the same position as it was when it was deposited. He 

 turtJier assumes that the strata in question are of lacus- 

 trine origin, and in his first paper he draws a vivid picture 

 of the "lake" filling up the Dei^vent estuary. Assuming 

 that Mr. Johnston's hypothesis be correct, it requires — 



(a) That the valley of the Derwent existed almost in 

 its present shape jDievious to the deposition of the leaf bed 

 series. 



(I)) That the strata deposited in this lake gradually 

 thin out towards the hill ranges on either side. 



The first proposition assumes that the deep canyon of 

 the Detrvvent was completed to about the present day level 

 in Pre-Eocene times (4). In other words, that the greatest 

 portion of the Derwent valley is geologically of very old 

 age (5). This is a very fascinating theory, but it mainly 

 depends on the age of the leaf bed series. There is every 

 reason to believe that the Tertiary strata of Australia are 

 much younger than Eocene; in fact, it is more than pro- 

 bable that they are not older than Miocene. Natui^ally 

 this would greatly reduce the age of the valley in which 

 the lake was whose waters deposited, the leaf beds- This 

 is a question which cannot be decided for the present, and 

 w'e have to leave it an open one, but I do not think that 

 the Derwent valley dates back as far as the Upper Cre- 

 taceous period. As regards the second proposition, we 

 should expect tliat the leaf bed series gi'adually gets 

 thinner towards the shore line of the old lake, which 

 naturally must be somewhere along the Mount Nelson 

 ridge, for instance, on the western side. Though I ob- 

 served the leaf bed series on the lower part of the new 

 road to Mount Nelson, I have net been able to ascertain 

 whether thev comply with this requirement of the lake 

 hypothesis or not. If anything can be said from the 

 rather unsatisfactoi*}' outcrops it is this, that the thickness 

 of the leaf beds is not reduced towards the hillside. 



On the other hand, Mr. R. M. Johnston's own sections, 



(4) For the sake of argument I accept the view that the Leaf Beds 

 are of Eocene age, though according to more modern researches this 

 view is no longer tenable. 



(5) Accepting the present view of the Eocene age of the Le-af Beds 

 and the hypothesis as to their origin, the Derwent Valley, at least that 

 portion above the present sea-level, would be of Cretaceous age. 



