Centre Valley Tin Ore 



will assume the gaseous form where space will allow, 

 tending to force their way into any cracks which may 

 occur in the enclosing rocks, upon which they may act 

 chemically. As they reach cooler and cooler parts of 

 the rocky cover, they will deposit various mineral 

 substances. 



This carriage of minerals as heated vapours, largely, 

 in all probability, by the agency of superheated steam, 

 is technically known as pneumatolysis, and it is believed 

 to have given rise to a very numerous class of mineral 

 veins. 



Later in the history of the plutonic rock, when the 

 temperature has fallen considerably, hot water and hot 

 acid liquids take the place of the hot vapours, and 

 continue the work of concentrating the mineral matters 

 originally disseminated throughout the magma. This 

 process is known as hydatogenesis, and is doubtless 

 responsible for many mineral deposits. 



To return now to the tin-bearing veins of the Granite 

 Mountains, we must first consider their character and 

 contents, and their relationship to the granite and to the 

 gneiss which surrounds it, forming part of the original 

 cover under which it solidified. 



The granite where we first encountered it in the 

 course of the stream was, as we have already stated, 

 very fine-grained, but this was merely a marginal phase 

 resulting from the rapid cooling of the edge of the mass. 

 The typical granite, remote from its edge, consisted of 

 quartz (SiO 2 ), Orthoclase Felspar (KAlSi 3 O 8 ), Oligoclase 

 Felspar (CaAl 2 Si 2 O 8 . 3NaAlSi 3 O 8 ), and Biotite Mica 

 (K,H) 2 (Mg,Fe) 2 (Al,Fe) 2 (SiCg 8 , with a few small cry- 

 stals of tin-stone (SnO 2 ), Apatite (3{Ca 3 (PO 4 ) 2 }CaF 2 ), 



283 



