354 PROCEEDINGS OF SECTION C. 



Freiberg (Saxony). The late Professor Dr. v. Groddeck 

 wrote a very interesting paper on the topaz discovery, which 

 he valued very highly, copy of which was also laid before 

 the Royal Society of Tasmania, and, I believe, excited great 

 interest. This paper gave rise to a scientific dispute between 

 Dr. Schroder and Professor v. Groddeck about the locality 

 from whence this specimen came, and the former maintained 

 that it had come from the " Schneckenstein," in Saxony, and 

 was by mistake ascribed to Tasmania. These two formations 

 must tlierefo2'e bear a close resemblance to one another, and 

 future exploration in our locaUty will be a matter of great 

 interest. 



At the present we have already done something in this 

 direction. In the porphyry dyke, on the same line Avhere 

 the topaz rock was found in the early days, in the alluvial 

 drift already referred to, striking nearly N. and S., similar 

 stone has been found in situ of considerable value ; but, not 

 only does the topaz rock with tin ore occur there, pure and 

 simple, but the porphyry also contains small crystals of the 

 same mineral, and I have no doubt the same will apply to 

 nearly all the jiorphyry dykes in the district. One remark- 

 able feature presents itself, and that is, none of these dykes 

 keep stanniferous after passing a certain line, beyond whicli 

 the porphyry changes its whole character, both in hardness 

 and its outside appearance. On the North Valley Road, not 

 half a mile from the centre of the tin-ore formation, all the 

 porphyry dykes are barren, and look quite different, and at 

 first sight no one would think they were the continuation of 

 the dykes crossing the stanniferous formation. One of these 

 dykes shows large cubical crystals of iron pyrites, and is 

 besides very dense and hard ; other dykes have other 

 peculiarities, but they are all barren outside the stanniferous 

 zone. 



Returning to the peculiar formation of the drift, we ha\e 

 to distinguish the diluvial from the alluvial, i.e., the older 

 waterworn (rounded) wash from the newer, which shows 

 sharp edges, or nearly so. Whether this difference is to be 

 accounted for by age alone, or whether the water action has 

 been more active in one place than in the other, is not very 

 easy to decide ; but I do not think there can be any doubt 

 that the Mount has presented different configurations during 

 its existence. 



How could it be possible, for instance, to deposit any 

 material upon an incline of 40° and more without precipi- 



