190 



porphyry in every respect, there can be no doubt that the 

 Tasmanian rocks are also topaz-quartz-porphyry, more especi- 

 ally after M. Schroeder has made us acquainted with the 

 topaz-quartz-porphyry of the Schueckenstein. 



It appears, therefore, as if in the tin district of Tasmania 

 (as in the Schueckenstein in the south-west of Saxony) a 

 transformation of the rocks with topaz had taken place 

 during the formation of the tin deposits. This is a pre- 

 viously unknown geological process of the highest interest, 

 since it introduces new views as to the origin of the tin 

 deposits. This process is probably not so isolated as would 

 at present appear. The discovery made by M. Schroeder is 

 the more interesting, as many pseudo-morphoses from topaz 

 to other minerals are known, but no change of another 

 mineral into topaz. 



On the Schueckenstein the transformation with topaz 

 occurs, according to M. Schroeder, by the topaz replacing 

 both tourmaline and feldspar. Such pseudo-morphoses are 

 not directly traceable iu the Tasmanian specimens now before 

 me ; but in them pseudo-morphoses from quartz to topaz 

 are evident. This must aid considerably in increasing our 

 knowledge of the appearance of topaz. In a block of about 

 5 cm. the inner portion consists of transparent crystalline 

 quartz, with an admixture of dark grains of tin. Towards 

 the outer portion of this block many irregularly-disposed 

 columns or cryssals are seen. Only a few of these show the 

 ordinary structure and form (oo E, E. — E.) of ordinary 

 quartz. The majority have a rough surface, are rounded at 

 the' angles, and are coloured pale-white. On breaking these 

 crystals, as a rule, a perfectly transparent nucleus of quartz 

 is seen, showing an uneven "fracture. This nucleus is sur- 

 rounded by a pale-white mass, which, on examination with a 

 lens, appears partly homogeneous, partly delicately striated. 



This mass is topaz, as is shown by an analysis made by Mr. 

 Sommerlad. The material for this was not to be obtained 

 absolutely pure, on account of the small quantity at our 

 disposal, and of the firm concretion of the topaz with quartz 

 and microscopic grains of tin. 



The analysis gave — 



Si- s 56-32 



AL'O* ... 35-91 



-£/ 10-68 



Ca. 0-03 



Sn. 0" 2 , 42f 



105-36 



tAs during the principal analysis the exact amount of tin present was not 

 ascertained, other specimens, as pure as possible, were selected for anotheranalysis. 

 These seem to have contained more microscopic tin grains than the first specimens. 



