106 SECTION AT ONE TREK POINT, 



R.S. TAS. 



siderable pox'tion of the latter must have been destroyed, 

 and are now replaced by Basalt (13). 



This is quite in harmony with the facts of volcanic 

 activity. Before the eruption of the magma took place 

 a considerable portion of the overlying strata is blown off 

 by explosions. It is vei^ probablej that the peculiar mass 

 of leaf bods above mentioned represents a portion of the 

 destroyed leaf beds which fell back. 



As the isolated patch of ve«iciilar Basalt observed in 

 the northern portion moves to a higher level than it is 

 now, the boundary line between Basalt and leaf beds must 

 have risen very rapidly towards north. 



This indicates the form.er existence of a cauldron-like 

 hole, which was subsequently filled with Basalt. In other 

 words, the Basalt formed originally a cone, a great portion 

 of which was subsequently destroyed. 



We can now trace the history of the events which 

 eventually resulted in the present features with the 

 greatest acciu-acy, but for the moment we will refrain from 

 expressing a view as to the geological time when they took 

 place- In order to make the sequence of events clearer, 

 we will not work backwards in descending order, but work 

 upwards from the earliest event we are able to trace. On 

 the supposition that Mr. R. M. Johnston's lake theory is 

 correct, and that tbe leaf beds are practically in the same 

 position now as they were when deposited, we have the 

 following sequence of events : — 



1. The formation by erosion of the Derwent estuary to 

 nearly its present depth. This was folkiwed by the forma- 

 tion of shore deposits, beginning with 



2. Sandstone, followed by the deposit of 



(15) I may be permitted to point out still anotuer difficulty; suppos- 

 ing that tlie "amount of the downthrow is more than 8o feet, in other 

 words tliat tlie northern part of the section does not reprr->?ent the 

 direct' continuation of the southern one. Tliis would naturally mean 

 that the portions of the section on either side of tlie fault are not 

 contemix>raneous, as lie assumed, but tlmt the southern jjortlon is 

 older than the northern one. In an undisturbed section we would 

 have, then, the following sequencer- 



Northern portion. 



Southern portion. 



In tills case the mass of strata removed above those that are now 

 to be seen would be far greater than here assumed ; aU the strata, (4), 

 f5> and (6), and a great part of (5) would have boon destroyed. 



