164 
On slicing the nodules a number of empty cavities occur 
n the face and are distributed throughout the nodule. 
seen to have crystallized in well-developed crystals. 
Numerous sections are shown. as hexagonal or rhombic. This 
idiomorphism of the quartz is the most striking characteristic 
of the slides. avri akan the tourmaline is present as 
grains moulded on the quartz. In some cases the moulding 
is developed as to yield a rude type of poikilitic fabric. 
to light- brown ye n any one ie 3 the dioi TE 
tion may be canaidorable this variation is, usually, 1rregu- 
larly developed im patches. The colour may also vary 
zonally. 
The origin of these Tasmanian nodules is not as clearly 
demonstrable as of those already described 
ro 
Willoughby, in which the process of pneumatolysis is actually 
in progress. The evidence so far revealed, however, 5. 
that the Tasmanian nodules are essentially of miarole origin. 
On this view come into line :— 
(1) mee grin. idomorphism of the sonia qu 
(2) The presence of a central cavity in many of 
soda ya 
(3) The prs general occurrence of canlara either 
as a trace or in oe amount, and, in some 
nodules, of fluori te 
id The nodules are thus poran to a comparatively ren 
E stage in the crystallization of the aplitic magma, ra her ks : 
presenting early segregation prod e associatio ; 
@) and (3), noted above, are regarded as strong eviden E 
their late miarole-pneumatolytic origin. The o thus ou 
Willoughby examples, is closely analogous to it. 
SUMMARY. i 
n 
quartz- oan nodules are developed i 
T e nodules i 
P. n Mere microcline and alb nate 
quomm replacing io. felspar, and is uate d pue 
m^ The mode of origin of the nodules is discussed, and 
x that they cannot represent segregati 
from the inm itic magma. 
ons of earliet — ir 
