159 
Stanley River District, and has examined in more detail 
those occurring in the Mount Heemskirk acid intrusives.(? 
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, ©) *'The 
are due to the operation of magmatic differentiation in the 
orginal magma, the minerals now forming these nodules 
ae gradually segregated and solidified as cooling pro- 
e 3» 
Apparently, similar nodules are developed in aplites 
associated with the granitic batholith of the Elkhorn District, 
ontana, as described by Barrell.ó Knopf also describes. 
nodules from aplite in the same region, but south of Montana 
city. These aplites are regarded as differentiates of the 
Same batholith of quartz-monzonitic type, common to the 
n 
that they are segregation products of earlier crystallization 
cept Mic 
that the nodule has developed in situ. Ith n mentioned 
above that the nodules are almost entirely relegated to the 
M red variety of the main apli imilar circum- 
urrences, where Water- 
house, in referring to their occurrence, says, ''In the field 
©) L. L. Waterhouse: Bull. No. 15, Geol. Surv. Tas., 1914, 
a L. L. Waterhouse: Bull. No. 21, Geol. Surv. Tas., 1916, 
p. 71. 
(9 Loc. eit., p. 28. 
(9 J. Barrell: 22nd Ann. Report U.S.G.S., 1901, pp. 542, 543. 
(? A. Knopf: Bull. 527, U.S.G.S., 1913, pp. 34, 35, 53. 
(9 Loc. cit., p. 99. 
