36 DISCUSSION ON NOTES ON THE MOUNT LYELL DISTRICT. 



fissures were common in lodes. The difficulties of imagining 

 that a lode fissure could be kept more or less open disap- 

 peared when it was remembered that fissures were generally 

 uneven, and that " faulting," even to a very slight extent, 

 would result in a series of more or less connected but irre- 

 gular cavities. That many lodes were lines of fault was well 

 known. He instanced the case of the Tasmania reef at 

 Beaconsfield, which crosses highly inclined and distinctly 

 bedded strata almost at right angles. In the drive on the 

 lode westward at No. 6 level a band of black rock is found on 

 both sides of the drive for some distance from the shaft. 

 On one side of the lode white sandstone then makes its 

 appearance, but the black still continues on the other side, 

 and it is not till a further distance of 104 feet has been 

 passed over that the white sandstone comes in on it, thus 

 proving a throw of the beds of the country rock of that 

 amount. 



Mr. E. M. Johnston expressed general approval with the 

 criticism of Mr. Montgomery, and expressed regret that Mr. 

 Power had not confined^ his attention to one of the many 

 important subjects introduced and treated it in a more 

 scientific manner. 



Mr. W. F. Ward, A.R.S.M., Government Analyst of 

 Tasmania, said : — Mr. Power, in his notes on the Mount 

 Lyell district, takes exception to the supposition that 

 " many of the Tasmanian mountains are due to the shrinkage 

 of the earth's crust," although he admits that " some slight 

 puckering of the surface may be due to this cause." In so 

 doing he joins issue with the great majority of geologists; 

 for, as Geikie, in summarising the causes of upheaval and 

 depression, says : — " With modifications, the main cause of 

 terrestrial movements is still sought in secular cooling " (and 

 consequent contraction). Confining our attention to the 

 Tasmanian mountains, we may say that the highest peaks are 

 one mile in height, the diameter of the earth 8,000 miles ; 

 the whole of this cannot therefore be called more than a 

 " slight puckering," and it would on a 2ft. globe be represented 

 by the thickness of thin foreign note paper. As regards the 

 alleged inadequacy of the shrinkage to produce this com- 

 paratively slight effect, Mallet, the great authority on this 

 branch of the subject, estimates that the diameter of the 

 earth when liquid was at least 189 miles more than it is 

 now; this implies also a shrinkage of the circumference to the 

 extent of nearly 600 miles, so that we seem to have a very ample 

 margin, sufficient to account not only for Tasmania's com- 

 paratively small hills, but for such enormous rock masses as 

 the Himalayas and the Andes. Not satisfied apparently with 

 this, however, Mr. Power invokes other agencies for the for- 

 mation of mountains, and says : — 



