158 



and, as a consequence, stand out in relief from the aplite in 

 which they are enclosed. They occur apparently quite 

 irregularly arranged in the mass, but appear with few excep- 

 tions to be confined to the finer-textured variety of the aplite. 



In section, as seen on the rock face, they appear more 

 or less elliptical, although some are really hexagonal. The 

 form taken by the majority of the nodules is, however, an 

 ellipsoid. In size they are slightly variable, but the greater 

 number have diameters, approximately, of 2 in., or slightly 

 less. 



A number of thin sections of these nodules was cut, and 

 microscopical examination showed them to consist, essenti- 

 ally, of quartz, felspar, and tourmaline. The nodules show 

 the general texture of the surrounding aplite. Tourmaline is 

 abundant, and is characteristically developed in the act of 

 replacing the microcline and albite felspar. All stages of 

 replacement can be traced, from the initial stages to complete 

 replacement. Minute arms of tourmaline stretch, at intervals, 

 through the felspar, isolating portions of the one felspar from 

 each other, in just such a way as to prove the development 

 of tourmaline from the felspar. The tourmaline shows strong 

 pleochroism, and is of blue colour of varying shades, tending 

 to brownish-green. This is the blue aluminous tourmaline 

 characteristic of felspar derivation. (tf Minute amounts of 

 muscovite may be associated with the tourmaline. 



The process of replacement described is well shown in 

 the microphotos which accompany this paper. Quartz is pre- 

 sent in clear grains with minute inclusions, and the felspar 

 still unreplaced is heavily dusted with kaolin. Some quartz, 

 especially towards the centres of the nodules, is probably 

 secondarily produced during pneumatolysis. In some nodules 

 the amount of replacement of felspar grains by tourmaline 

 becomes more complete as the centre is approached. At the 

 centre the remnant of a felspar grain may only be represented 

 by a shred at the periphery, or a shred in the interior of the 

 tourmaline grain. The proportion of quartz in such cases 

 may increase at the centre, suggestive of silica being derived 

 from the felspar interaction. 



Nodules somewhat similar to those just described have 

 been previously noted by investigators of the Tasmanian 

 Geological Survey. Waller ( 2 ) noted their occurrence in an 

 aplite from Mount Heemskirk, Tasmania. More recently 

 L. L. Waterhouse has also described similar nodules in the 



(DC/. Mem. Geol. Surv. Eng. and Wales, 1909, p. 65; 

 Scrivenor : Quart. Journ. Geol. Soc, vol. lix., 1903, p. 151. 



(2) Waller : Report on the Tin Ore Deposits of Mount Heems- 

 kirk, Geol. Surv. Tas., Sept., 1902, p. 4. 



