34 THE MINERALS OF TASMANIA. 



Company, near Queenstown. It sometimes occurs in the 

 veinstone of metallic lodeSj but is not known in large quan- 

 tity in this connection. 



65. Cassiterite (Dioxide of Tin). 



As is well known, this is the common ore of tin, which 

 is practically the only commercial one of the metal. It 

 has a wide geographical distribution, but in few localities 

 doss this ore occur in such quantity as to be of economic 

 importance. It has been fully established that in all 

 parts of the world there exists a striking connection 

 between the occurrence of tin ore and the intrusion of an 

 acid plutonic magma, resulting in the formation of 

 granite and kindred rockmasses. It has further been proved 

 that such occurrences are closely associated with a peculiar 

 group of minerals characterised by the presence of fluorine, 

 boron, phosphorus, and arsenic. This connection is so 

 pronounced, that it was early in the last century that 

 Daubree formulated the now almost universally-accepted 

 theory that the tin was brought up from plutonic depths 

 in the form of a volatile fluoride. The emanations were 

 probably at a temperature above critical point, or in the 

 permanently gaseous state, in which condition they would 

 be the means of producing an alteration in the rocks 

 which has been termed pneumatolytic. It is believed by 

 Yogt, who has carefully studied the genesis of ore- 

 deposits, that the formation of tin-lodes probably started 

 before the absolute cooling of the granite. It is also 

 supposed that many tin-bearing deposits have been favour- 

 ably effected by the agency of permeating thermal waters. 

 It has been shown that cassiterite is sparingly soluble in 

 water containing alkaline carbonates, and even slightly 

 in distilled water at 80° C, which solubility is much 

 increased by the presence of sodium fluoride. It has been 

 found that a deposit of opaline silica formed from a warm 

 spring at Selangor contained on analysis 0'5 per cent, of 

 stannic oxide. There are records which show that in Corn- 

 wall antlers of the red deer have been found in the tin- 

 bearing gravels partially transmuted to, or impregnated 

 with cassiterite. In the same way beautifully-developed 

 crystals, mostly Carlsbad twins, of orthoclase have been 

 altered or pseudomorphed to the same substance at the 

 Wheal Cootes and other mines in the same Old World tin- 

 mining locality. In this island much the same thing has 

 occurred, although to a somewhat less pronounced extent, 

 in the tin-bearing granite of Mt. Rex, where the outline 



