334 



(b) They are of secondary origin, and represent the 



albitization of original microcline dyke-rocks, this 

 albitization being accomplished by magmatic soda- 

 rich solutions. 



(c) They represent the "end product" and final differ- 



entiate of the residual magma, and are therefore 

 directly related to the potassic-aplites. 



(a) An immiscible phase of the liquid residual magma. — 

 Daly,( 21) Grout, ( 22) and others have resorted to liquid immis- 

 cibility to explain certain types of differentiation. 



It must be admitted that the evidence for the separation 

 of liquid phases in igneous magmas has not yet been clearly 

 demonstrated, nor has the extensive experimental work on 

 silicate-melts given any indication of such a process. 



For the Willoughby albitites it is thought that this 

 mechanism is untenable, for homogenous rock masses abound 

 in which all minerals herein concerned, viz., quartz, micro- 

 cline, and albite, are associated in a wide range of mixtures. 

 It is to be noted here, however, that the objection raised by 

 Bowen (23 ) that the formation of a monomineralic rock is 

 generally impossible by liquid immiscibility, owing to the fact 

 that this would necessitate its crystallization at its true melt- 

 ing point — i.e., far above the temperature of the magma, say 

 for albite, 1100° C. — ignores the possibility, theoretically, 

 of albite and a volatile mineralizer (e.g., water) separating 

 as a liquid phase, in which case the reductio ad absurdum 

 argument fails. 



(b) Albitization of original potassic rocks. — This view 

 immediately admits the albitites to be of secondary origin and 

 the process of albitization to have been produced by magmatic 

 sod a- rich solutions. 



As far as the writer is aware, there is no evidence of 

 albitization of the surrounding granite with which the albi- 

 tites are in contact. Nor does the texture of these rocks 

 suggest such a replacement. Both occurrence and texture are 

 strongly against their derivation from original potassic-aplites. 



(c) The albitites represent the final differentiate or end 

 product of the residual magma. — The writer is of the opinion 

 that the albitites represent the final differentiate of the residual 

 magma. 



The intimate relationship of the potassic-aplites and the 

 albite-pegmatites is indicated by the presence, in each, of the 



(21) R. A. Daly: Igneous Rocks and their Origin, p. 226. 



(22) F. F. Grout: Econ. Geol., 1918, p. 18-5. 



(23) N. L. Bowen : Journ. Geol., Dec, 1915 (Supplement), 

 p. 80. 



