836 BROWN & HERON : GEOLOGY AND ORE DEPOSITS OF TAVOY. 



Pneiunatolysis. — Various writers have used this word in many 

 different senses and we believe that a great deal of confusion of 

 thought has arisen through their readers failing, often through no 

 fault of their own, to comprehend the exact shade of meaning of 

 the writer, or to appreciate what processes or phases he intends 

 the word to cover. Rastall thinks that the use of the word is need- 

 less in connection witli the processes of differentiation which take 

 place in the crystallization of the last residues of an acid magma, 

 in that it seems to imply something unusual, abnormal and out of 

 the common order of events. He prefers to divide the different 

 phases of the processes as follows : — 



1st phase. — Concentration within the magma of the metallic 

 constituents in combination with the volatile elements. 



2nd phase. — Separation from the crystallizing granite of the 

 compounds thus formed, and escape of the same through 

 fissures. 



3rd phase. — Chemical reactions between the compounds in the 

 escaping gases, or solutions, leading to the formation of 

 crystallized ore and gangue minerals. 



" This kind of differentiation is perfectly normal ; its final 

 results depend mainly on the extent to which highly 

 volatile compounds and water were present in the original 

 magma. The more of these are present the lower will be 

 the freezing point of the final product. The effect of al- 

 kaline tungstates in lowering the freezing point of acid silicate 

 melts, and thus enabling quartz and felspar to crystallize, 

 has long been known and used in penological and minera- 

 logical research." 1 



The subject has been discussed in another connection by Grafton 

 and McLaughlin, who point out that Runsen, in advancing the term, 

 employed it in a very limited sense to the action of volcanic gases 

 in and near fumaroles, and that its modern usage as generally under- 

 stood to suggest destructive action by corrosive mineralizers such as 

 lluorine. chlorine, boron, sulphur and phosphorus, with the production 

 of minerals like tourmaline, topaz, fluorite and scapolite, is a 

 liberal extension of its original narrow meaning. 2 



1 R. H. RaBtall, Grot. Mag., 1918, p. 369. 



* L. C. Grafton and D. H. McLaughlin, " Further Remarks on the Ores of Engles, 

 California," Econ. Qeol.,Vol XIII, pp. 81-99, 1918. 



