165 
IV. A mode of origin, in situ, is suggested which is in 
harmony with the evident pneumatolytic replacement that 
has oecurre 
Y, As denoting their manner of origin, it is suggested 
that the name ‘‘Pneumatolith” be attached to such pseudo- 
segregations occurring in igneous rocks an which owe their 
existence, primarily, to pneumatolytic processes. 
. The evidence of the Tasmanian nodules, while not as 
clearly delineated as in the Cape Willoughby examples, i 
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 klagen related to the origin described for the 
Cape Door nodules. 
The author is indebted to Mr. W. R. Browne, B.Sc., 
for helpful | discussion during the preparation of this paper. 
DESCRIPTION OF PLATES. 
Prare XXIII. 
Fig. 1. Photograph of a typical quartz-tourmaline ghee 
The piod ellipsoid shape of the nodule is apparent. A portion 
of the aplite is attached to its upper rear surface. Natur 8L ei. 
Fig. 2. Section of a er aire a nodule pritet ro 
Sp aamont of feles by tourmaline. The tourmaline can be 
a network through the fe — ieri sections of ihe d one 
eters. 
fiber from each pe om Magn 5 diame 
Prate XXIV. 
ng 1. pe section. The clear areas are quartz. 
A 
Magn gn. E 45 diameter 
Fig. 2. rtion sige fig. 1 enlarged to show the replacement 
of the Pos: by the strings of tourmaline. Magn. x 80 diameters. 
