165 



IV. A mode of origin, in situ, is suggested which is in 

 harmony with the evident pneumatolytic replacement that 

 has occurred. 



V. As denoting their manner of origin, it is suggested 

 that the name "Pneumatolith" be, attached to such pseudo- 

 segregations occurring in igneous rocks and which owe their 

 existence, primarily, to pneumatolytic processes. 



VI. The evidence of the Tasmanian nodules, while not as 

 clearly delineated as in the Cape Willoughby examples, is 

 strongly suggestive of miarole origin. Their formation is 

 then referable to a late stage in the crystallization of the 

 magma. The origin is distinct from the hypothesis of "segre- 

 gation," and is closely related to the origin described for the 

 Cape Willoughby nodules. 



The author is indebted to Mr. W. R. Browne, B.Sc, 

 for helpful discussion during the preparation of this paper. 



DESCRIPTION OF PLATES. 



Plate XXIII. 



Fig. 1. Photograph of a typical quartz-tourmaline nodule. 

 The general ellipsoid shape of the nodule is apparent. A portion 

 of the aplite is attached to its upper rear surface. Natural size. 



Fig. 2. Section of a quartz-tourmaline nodule showing the 

 replacement of felspar by tourmaline. The tourmaline can be seen 

 as a network through the felspar, isolating sections of the one 

 felspar from each other. Magn. x 45 diameters. 



Plate XXIV. 



Fig. 1. Another section. The clear areas are quartz. 

 Magn. x 45 diameters. 



Fig. 2. A portion of fig. 1 enlarged to show the replacement 

 of the felspar by the strings of tourmaline. Magn. x 80 diameters. 



