BY D. MAWS0N. 605 



The discovery of a small quantity of fluorine in the arf vedsonite 

 is of special interest in view of recent conjectures by petrologists, 

 suspicion being raised that perhaps traces of fluorine may be an 

 invariable constituent of amphibole. 



Another case has been brought to light of the occurrence of 

 alkali-amphibole and related pyroxene in pegmatite veins in 

 association with fluorite, titanium minerals and zircon. In the 

 author's opinion, too much stress may be placed on deductions 

 drawn from this fact as to the effect of mineralisers, other than 

 water, on the resulting crystallisations. It is, however, incon- 

 trovertible that the presence of water and to a less extent, 

 perhaps, that of fluorine, are necessary for the formation of 

 amphibole in preference to pyroxene. This has been demon- 

 strated by many writers.* 



The occurrence of segirine in excess of arf vedsonite in the more 

 local veins described in this paper, and the abundance of the 

 latter in the wider and more continuous veins, especially in the 

 central part in positions where hydrous action is demonstrated 

 to have had supreme effect, afford additional confirmation of 

 the correctness of this conclusion. 



The frequently noted intergrowthsf of pyroxene and amphibole 

 would on this determinant alone be difficult of explanation, unless 

 it is assumed that a hydrous magma had been replaced sub- 

 sequently by anhydrous conditions or vice versa. 



In these veins examples are common where a considerable 

 range in composition is experienced from the kernel outwards, 

 both in the case of pyroxene and amphibole. In some, the 

 valiant was found to consist only in the state of oxidation of 

 the iron contents, and may indicate the introduction of periods of 

 greater and less intensity of oxidation or even reduction. 



* Note especially " On the Genesis of Riebeckite and Riebeckite Rocks," 

 by G. M. Murgoci, Amer. Journ. Sci. Aug. 1905, p. 133. 



f Primary intergrowths of pyroxene and amphibole, which could be 

 definitely determined as such, were not met with in any of these veins. 



