xlviii 



These suggestious, however, he merely 

 advanced as speculations for the further 

 consideration of the physicist and the 

 petrologist. 



The paper was illustrated with a fine 

 series of photographs. 



DISCUSSION. 



Mr. Thos. Stephens, M.A., F.G.S., said 

 the remarks of Messrs. Twelvetrees and 

 Petterd on the main constituents of the 

 Tasmanian " dolerite," with their acces- 

 sories, and in the intersertal and ophitic 

 structure of different varieties of the rock, 

 wex-e an admirable summary of results of 

 detailed microscopic examinations which 

 have already been laid before the Society, 

 and will be very useful to all who are in- 

 terested in this important branch of geo- 

 logical research. The authors have amply 

 justified their use of the term " dolerite" 

 rather than " d^iabase," which latter term 

 some of us still prefer to use for the rocks 

 in question, and their work is immensely 

 valuable to all who study the geological 

 history of Tasmania. 



Mr. Stephens said the development 

 and surroundings of the vast masses 

 of dolerite, or diabase, in the central 

 plateau must, of course, be studied 

 in the field before any conclusions 

 as to their origin can be accepted as final ; 

 and we must not begin by regarding them 

 as " occupying the whole upland area of 

 the Central Tiers." When referring to my 

 own knowledge of the central plateau, I 

 haA-e preferred to speak of it in the follow- 

 ing terms: — "The central plateau is not, 

 as is generally supposed, a vast boss of 

 ancient volcanic rocks, but rather a ring 

 of massive dykes and caps of diabasic 

 greenstone or dolerite, with intricate re- 

 ticulations. These traverse all the rocks 

 of pre-Tertiary age, and are interspersed 

 with sheets of more recent basaltic lavas." 

 I may also say that, at numerous points 

 over the vv'ide area of the plateau, there 

 are outcrops of permo-caiboniferous rocks, 

 as well as some remains of the upper coal 

 measures, which will have to be taken into 

 account when the geological history 

 of the whole area is I5eing investigated. 

 I think I may say that we all heartily 

 congratulate 'Mr. Twelvetrees on his ap- 

 pointment to the post of Government 

 Geologist. If I may judge from my own 

 experience o" oflicial life, he will hardly 

 find it possible to give much special atten- 

 tion, in the near future, to what is evi- 

 dently to him a very favourite branch of 

 scientific research ; but his opportunities 

 of geological observation will be largely 

 extended, and it goe^s without saying that 

 he will make good use of them. 



Mr. Stephens said Mr. Johnston's paper 

 comes in very opportunely in connection 

 with the subject of that of Messrs. Twelve- 

 trees and Petterd. Before dealing with 

 the general question, he makes some re- 

 marks which are specially interesting to- 

 myself. In the true spirit of scientific 

 inquiry, which rigidly subordinates theory 

 to ascertained facts, he mentions that he- 

 has notified the opinions v.hich he formerly 

 held as to the relations of the igneous and 

 sedimentary rocks between Blackman's 

 Bay and Passage Point. I have always 

 regretted that, on a point which is inti- 

 mately connected with similar phenomena 

 all over the island, I found myself quite at 

 variance with one who has done more than 

 any one else for the development of the- 

 geological history Tasmania, but this 

 difference of opinion no longer exists. The 

 remarks of Professor David, quoted by 

 Mr. Johnston, to the effect that gigantic 

 masses of igneous rock extensively de- 

 veloped along the coast lines of Southern 

 Tasmania "are in reality sills, rather than 

 old lava flow," are partly based on the 

 evidence of certain sections which I 

 showed him on his first visit to Tasmania ; 

 but I am inclined to think that by " old 

 lava flows " he means such dyke cappings 

 of vast thickness as might have been 

 extensively denuded, leaving only the- 

 lower portions still of great thickness^ 

 These, having cooled slowly under the 

 pressure of the upper layers, would, 

 it might be inferred, exhibit the holo- 

 crystalline structure so characteristic of 

 true "sills." This is what Mr. Johnston Ij 

 himself seems to suggest, when speaking| 

 of the massive caps to which his paper-*' 

 refers, and it is one of the theories which 

 must be seriously considered before we can. 

 come to any definite conclusion on the 

 subject. In connection with the question 

 of the part which denudation plays in 

 determining the physical features of land 

 surfaces, I may cite the instance of the 

 grand canon at Flagstaff, in Arizona, 

 which has been excavated by ordinary 

 natural agencies to a depth of 6,370ft., 

 with a width of 13 miles — a cavity which 

 would hold the biggest mountain of Tas- 

 mania, and have plenty of room to spare. 



At the conclusion of Mr. J. B. Walker's- 

 paper, " Early Cartography and the Terra 

 Australis Myth," which was illustrated by 

 some excellent prepared slides, Mr. R. M. 

 Johnston showed a series of geological 

 slides, illustrating intercalated "sill" 

 flows of igneous rocks, and also massive i 

 flows of the same rocks as at Cape Pillarij 

 and Tasman Island. f 



The President moved a vote of 

 thanks to the authors of the several 

 papers. The slides used were prepared by 

 Mr. J. W. Beattie, and the limelight 

 lantern was worked bv Mr. Nat Oldham. 



