PRESIDENTS ADDRESS — SECTION C. 75 



sea-level, it is evident that a subsidence of about 200 feet 

 would be necessary in order to bring the beds containing the 

 Tertiary fossil-wood down to their present level, on ithe 

 assumption that they were originally laid down at or above 

 sea-level. 



Whether or not the subsidence which took place during 

 the accumulation of the Tertiary beds of the Launceston basin 

 had any relation to the formation of Bass Straits is not at 

 present known. 



Mr. Johnston is of opinion that the depression which con- 

 tributed to form Bass Straits occurred during the later 

 Tertiary and earlier Post-Tertiary periods, in which case 

 Bass Straits were probably formed at a later date than that 

 of the eruption of the basalts of the Launceston basin. 



Before proceeding to summarise the evidence so far adduced 

 as to the possible relation of volcanic action in Eastern 

 Austraha and Tasmania to heavy sedimentation and oscilla- 

 tions of the earth's crust, it may be well for the author to 

 review briefly some theories which seem to offer a reasonable 

 explanation of this probable relationship. 



Assuming that the earth was originally formed from the 

 condensation of a nebula, Messrs. Nasmyth and Carpenter 

 have shown how a globe of molten matter like the earth or 

 the moon cooling in concentric shells from its surface inwards 

 would be subject to superficial outbursts of volcanic activity. 

 As the first crust (a) underwent solidification it would 

 undergo expansion, but, as there would be nothing above it 

 but the atmosphere to resist this expansion, no considerable 

 disturbance of the earth's crust would ensue. In the second 

 stage of coohng, owing to further loss of heat, the first-formed 

 crust would contract, and this contraction would be resisted 

 by the expansion which would simultaneously be se#up in the 

 zone (b) undergoing solidification, and therefore expansion 

 immediately under the first cooled crust (a). The result of 

 the opposing action of these two zones would be to exert 

 immense bursting force upon the outer zone («). The action 

 of the expanding zone (b) at this stage might be compared 

 to that of water bursting iron pipes consequent on its expan- 

 sion when approaching the freezing point, or to that of molten 

 bismuth when at the point of sohdifying, itbeinga well known 

 fact that that metal undergoes expansion when solidifying 

 to such an extent that if, when molten, it be poured into an 

 iron bottle so that the latter is completely filled, and the 

 stopper be screwed down to prevent its escape, the bottle will 



