1 



sonally the honor of acting as field-guide 

 at the time, were inclined to be of opinion 

 "that the gigantic masses of (jahhro" (i.e., 

 the diabase or dolerite of Professor Ulrich ; 

 Messrs. Twelvetrees, and Pettord, the 

 writer, and others), which are so extensively 

 developed along the estuary of the Derwent 

 as well as along the South-east Coast, in- 

 cluding Freycinet's Peninsula, are in reality 

 sills, rather than old lava flows as was for- 

 Tiierly contended by some.'' The latter part 

 of Professor David's remarks I have 

 italicised, as it is rather misleading if it 

 refers to the opinion entertained by myself 

 and other local geologists who may have 

 written about the massive diabasic rocks so 

 largely developed throughout the central 

 and eastern parts of Tasmania. 



There never was, to my knowledge, any 

 qiiestion at any time under discussion 

 among loc*l geologists as to whether our 

 massive diabasic intrusions — forming the 

 prominent features on our mountain caps, 

 tiers, and along our Eastern shores — were 

 originally erupted subaerial y as lavas, or 

 whether as colossal sills they were origin- 

 ally injected or intruded into strata lying 

 below the surface; the superincumbent 

 rocks which formerly enveloped their 

 mass, have been long since swept 

 away by subsequent denudation continued 

 for ages until now. Up to the time at 

 which Messrs. Twelvetrees and Petterd 

 commenced their splendid microscopic in- 

 vestigations of our igneous rocks — of which 

 their latest contribution read this evening 

 is, in itself, a rich mine of wide and valu- 

 able knowledge and logical deduction — 

 the local general geologists were not 

 possessed of the necessary data to enable 

 them to form conclusions that would be at 

 »11 satisfactory in a question of this kind. 

 Without a thorough survey of all 

 our igneous rocks— by such methods of 

 careful systematic microscopic examination 

 as are, now, so ably being carried 

 on by our own observers, Messrs. 

 Twelvetrees and Petterd — I do not think 

 any conclusions as to their exact mode of 

 origin can be of much scientific value. 



What, hitherto, specially attracted the 

 attention of local obseivers and of the 

 earlier geologists, Jukes, Selwyn, Milligan, 

 and Gould, as regards the greenstone 

 massts now capping permo-cirb. aod 

 mesozoic rocks on the Great Lake Plateau. 

 Ben Lomond, Mount Nicholas, Fingal Tier, 

 Mount WeJlington, and elsewhere was, 

 " We e they superimposed massive caps, 

 or weie they massive greenstone coies 



against the flanks of which the permo-carb. 

 mudstones and the mesozoic coal measures 

 rested ? " The economic importance of 

 this question is very great ; for if the 

 massive greenstones on top of Ben Lomond, 

 Mount Nicholas, are caps which may have 

 been fed by roots from below, we might 

 hope to follow our coal seams throughout 

 the areas, enveloped by these extensive 

 masses. But if these colossal caps are 

 themselves continuous through the permo- 

 carb. and mesozoic rocks as vast co-exten- 

 sive diabasic cores, then the limits within 

 which we may follow the coal seams on 

 their flanks will be correspondingly reduced. 

 This, and this aspect of the case a one, had 

 hitherto been the vexed question between 

 some of our Tasmanian geologists, and not 

 the newer aspect raised, viz., lava versus 

 sill, structure and mode of origin. 



Lava versus Sill Origin of the Higher 

 Mountain (Japs. 



Nearly two-thirds of the whole area of 

 Tasmania in its Midland and Eastern part 

 is occupied continuously or ramified by 

 masses of the diabasic greenstone lockd 

 which were erupted towards the close of 

 the mesozoic era. The great plateau of 

 the Lake Country alone is almost continu- 

 ously occupied by this rock for over one 

 thousind square miles Its outer edge to 

 the West, North, and East, forms pre- 

 cipitous tiers bordering the lower p ains 

 and generallv reaches a height of from 

 3000 to 400 J fee', and. in some cases, rising 

 to a height of over 5000 feet. 



The general thickness of the more charac- 

 teristic mountain caps of greenstone, as on 

 Ben Lomond and Mount Wei iogton, even 

 now, after ages of exposure to denvidation, 

 are from 1700 to nearly 3000 feet thick. 

 If we assume, for purposes of illus- 

 tration, a period of t^^o and a half 

 million years having elapsed since the 

 close of the mesozoic era; and that our 

 higher levels have been continuously ex- 

 posed to denudation for the whole of that 

 period — What would the extent of waste 

 rt-present in the destruction of the masses 

 of rock of whatever nature which weie 

 originally super-imposed upon them over 

 their existing high level limits ? ; and — 

 What a-e like'y to have been the character 

 of the rocks which have been wasted away 

 from above them ? 



The usual estimate of the rate of denu- 

 dation by atmospheric influences andgiavi- 

 tation is reckoned to be equal to a waste, 

 on the average, of one foot of rock in 3000 



