163 



the concentration of the active gases. Quartz would, of 

 course, be unattacked ; but silica would probably be released 

 in the interaction with felspar. With a still further reduc- 

 tion, the remaining gases would be dissolved or condensed. 

 The process would have initiated before final and complete 

 consolidation took place, and the accompanying excess of 

 alkalies, from the felspar interaction, would diffuse into the 

 still liquid residue, partly surrounding the gaseous bubble. 



For the Cape Willoughby quartz-tourmaline nodules the 

 following data are in accord with the hypothesis outlined in 

 the previous pages for their manner of origin : — 



(1) The pneumatolytic origin of the tourmaline. 



(2) The development of the nodules in, and their 

 practical relegation to, the finer-grained variety 

 of the red aplite. 



(3) The composition and texture of the nodule in 

 which the tourmaline is replacing the felspar is 

 identical with that of the associated aplite. 



(4) The general ellipsoidal character of the nodules. 

 As denoting their manner of origin, it is suggested that 



the name "Pneumatolith" be attached to such pseudo- 

 segregations occurring in rocks, and which owe their existence 

 primarily to pneumatolytic processes. 



From the published descriptions of the Montana nodules 

 previously noted, the exact relationship of the tourmaline 

 to the orthoclase felspar associated with it is not clear, but 

 the description of Barrell ( 16 ) is suggestive of the tourmaline 

 being of pneumatolytic origin. 



A study of the literature on the mode of occurrence of 

 tourmaline indicates that this mineral does, occasionally, 

 appear pyr ©genetically. This is especially so where it is 

 present as an accessory uniformly distributed through 

 granites or aplites. It is, therefore, possible that segregations 

 of quartz and pyrogenetic tourmaline can occur. 



Through the kindness of Mr. W. H. Twelvetrees, Govern- 

 ment Geologist of Tasmania, I have been able to obtain a 

 number of nodules from the Heemskirk District for micro- 

 scopical examination. A number of these have already been 

 described by Waller and Hogg. (17 ) 



The sections examined by the writer consist essentially 

 of quartz and tourmaline, felspar being absent. Macro- 

 scopically it has been recognized in one nodule, and is 

 represented by kaolin. 



(16) Loc. tit., Sup., p. 543. 



(n) Waller and Hogg: Papers Proc. Roy. Soc. Tas., 1902, 

 pp. 143-156. 

 g2 



