J64 



On slicing tiie nodules a number of empty cavities occur 

 on the face and are distributed throughout the nodule. 

 Whether these represent the spaces originally occupied by 

 felspar is not clear. Under the microscope, the quartz' is 

 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. Consequently 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. 

 Occasionally the tourmaline may also be developed in prism- 

 atic idiomorphs. The pleochroism of the tourmaline is 

 strong, the characteristic variation being from bluish-green 

 to light-brown yellow. In any one grain the colour varia- 

 tion may be considerable ; this variation is, usually, irregu- 

 larly developed in patches. The colour may also vary 

 zonally. 



The origin of these Tasmanian nodules is not as clearly 

 demonstrable as of those already described from Cape 

 Willoughby, in which the process of pneumatolysis is actually 

 seen in progress. The evidence so far revealed, however, is 

 that the Tasmanian nodules are essentially of miarole origin. 

 On this view come into line : — 



(1) The striking idomorphism of the constituent quartz. 



(2) The presence of a central cavity in many of the 

 nodules. 



(3) The very general occurrence of cassiterite, either 



as a trace or in appreciable amount, and, in some 



nodules, of fluorite. 

 The nodules are thus referable to a comparatively late 

 stage in the crystallization of the aplitic magma, rather than 

 representing early segregation products. The associations 

 (2) and (3), noted above, are regarded as strong evidence of 

 their late miarole-pneumatolytic origin. The origin thus out- 

 lined, while not identical with that described for the Cape 

 Willoughby examples, is closely analogous to it. 



Summary. 



I. The quartz-tourmaline nodules are developed in a 

 mass of aplite intruding the Cape Willoughby granite. 



II. The nodules consist essentially of quartz, tourmaline, 

 and felspar (microcline and albite). The tourmaline is in 

 process of replacing the felspar, and is evidently of pneumato- 

 lytic origin. 



III. The mode of origin of the nodules is discussed, and 

 it is shown that they cannot represent segregations of earlier 

 formed crystals from the aplitic magma. 



