159 



Stanley River District,^) and has examined in more detail 

 those occurring in the Mount Heemskirk acid intrusives.^ 4 ) 

 The descriptions given by these two investigators agree, fairly 

 closely, with the nature of the occurrence at Cape Willoughby. 



The presence of small amounts of cassiterite, the absence 

 of felspar from the centre, and the frequent presence of a 

 central cavity, seem to be the principal points of distinction 

 between the Tasmanian and Willoughby examples. 



In discussing the origin of these nodules, both writers 

 reach the conclusion that the nodules represent segregations 

 of quartz and tourmaline. To quote Waterhouse, ( 5 ) "They 

 are due to the operation of magmatic differentiation in the 

 original magma, the minerals now forming these nodules 

 having gradually segregated and solidified as cooling pro- 

 ceeded." 



Apparently, similar nodules are developed in aplites 

 associated with the granitic batholith of the Elkhorn District, 

 Montana, as described by Barrell. (^ Knopf also describes 

 nodules from aplite in the same region, but south of Montana 

 city. (7) These aplites are regarded as differentiates of the 

 same batholith of quartz-monzonitic type, common to the 

 Elkhorn and Helena Districts. The nodules contain quartz, 

 orthoclase, and tourmaline; but in neither case is the rela- 

 tionship of the tourmaline to the felspar clearly indicated. 

 Both Barrell and Knopf evidently regard them as segrega- 

 tions from the liquid aplitic magma; e.g., Knopf states, "The 

 tourmaline-quartz-orthoelase segregations are regarded as 

 imprisoned and congealed globules of this final differentiate." 



In the case of the Cape Willoughby nodules, the view 

 that they are segregation products of earlier crystallization 

 cannot be accepted. Microscopic and other evidence tends to 

 show that they are, indeed, strictly pneumatolytic products. 

 In the slides is to be seen the very act of replacement of 

 felspar by tourmaline. The texture and composition of the 

 nodule, apart from the presence of the tourmaline, suggests 

 that the nodule has developed in situ. It has been mentioned 

 above that the nodules are almost entirely relegated to the 

 finer-textured variety of the main aplite. Similar circum- 

 stances surround the Tasmanian occurrences, where Water- 

 house, in referring to their occurrence, says,( 8 ) "In the field 



(3) L. L. Waterhouse: Bull. No. 15, Geol. Surv. Tas., 1914, 

 p. 28. 



(4) L. L. Waterhouse: Bull. No. 21, Geol. Surv. Tas., 1916, 

 p. 71. 



(5) Loc. cit., p. 28. 



(6) J. Barrell: 22nd Ann. Report U.S.G.S., 1901, pp. 542, 543. 



(7) A. Knopf: Bull. 527, U.S.G.S., 1913, pp. 34, 35, 53. 



(8) Loc. cit., p. 29. 



