BY D. MAWSON. 603 



openings; in the latter case circulation was established with 

 distant sources. 



Veins occupied by bitumen distilled from the coal measures 

 below, form instructive cases and afford a key to other phenomena; 

 they are evidence of distant transportation and incorporation of 

 exotic material. 



The more local pegmatite veins could only have originated by 

 sweating from the sides. In the larger and more continuous 

 channels the residual gaseous and more liquid contents of the 

 solidif} 7 ing rock have collected largely in the same fashion, and, 

 while possessing a considerable freedom of circulation, have 

 crystallised out as a coarse-grained product. 



Another class of veins is distinguished as possessing indications 

 of viscous flow whilst in a semi-crystallised state; these latter are 

 finer-grained than the preceding, though noticeably coarser than 

 the syenite alongside. The explanation afforded is that this tj^pe 

 has probably been derived from material more in the nature of a 

 solution of small quantities of water and gases in magma, and 

 had commenced crystallising before equilibrium had been arrived 

 at, during viscous flow along developing passages. The coarser- 

 grained veins not showing flow-structure are held to be excellent 

 examples of water- and gas-solutions of the magma.* 



Examples intermediate between the latter two classes described 

 are common in the rock mass, and tend to bear out the conclu- 

 sions regarding pegmatites arrived at by Van Hise,f "there are 

 all gradations between heated waters containing mineral material 

 in solution and magma containing water in solution." 



In these Bowral veins the coarser types described under class 

 (b) are true pegmatites in every sense of the term; class (c) types 

 with their finer grain and flow-structure are more nearlj' related 

 to the aplites. 



Van Hisel brings forward solid arguments proving that peg- 

 matites may pass by insensible gradations to simple quartz reefs. 



* C. K. Van Hise, "A Treatise on Metamorphism," Mon.47, United States 

 Geol. Survey, p. 732. 



t Op. cit., p. 723. 

 t<>p. cit., p.724. 



