328 



quartz solutions in the associated vein did not completely 

 infill the cavities. 



V. The Nature and Composition of the White Pegmatite. 



The mode of occurrence of these dykes and veins has 

 already been described, including a brief macroscopic descrip- 

 tion of the various types. 



When these rocks were examined microscopically the pre- 

 dominant felspar was found to be albite, and the rock types 

 can now be classed as albitites. The varieties present are 

 quartz-albitites, muscovite-albitites, and an almost pure 

 albitite consisting practically of albite. This rock occurs in 

 veins associated with a quartz-albitite. 



• • 18° 



(a) Albitite. — D 40= 2*622: — Under the microscope this 

 rock is seen to consist essentially of albite. Accessories are 

 apatite, in hexagonal crystals; zircon, in idiomorphic prisms, 

 showing high polarization colours ; and rutile, usually in 

 prismatic forms. 



Muscovite is present in small tufts and is usually associated 

 with apatite, zircon, and rutile. The albite is usually sub- 

 idiomorphic to allotriomorphic. A curious mottled twinning 

 shows up in some sections. This has been described as 

 "chequer albite." ( 5 ) In other sections only well-defined albite 

 lamellae are present. 



A very small quantity of interstitial quartz is present in 

 the slide. 



The chequer structure is due to the presence of irregular 

 interpenetrating twin lamellae. No traces of a mottled char- 

 acter, however, are present on sections parallel to (010). This 

 structure was first described by Becke.^ Flett and Hughes 

 have noted its development in phenocrysts of volcanic rocks 

 associated with albite of the usual kind. 



In the albite rocks under consideration, there is no 

 evidence to suggest albitization of original microcline felspar. 



Some albite sections show a transition from normal albite 

 lamellae to the chequer t} 7 pe. Its occurrence, associated with 

 normal albite, both here and in the example given by Flett, 

 appear to negative any secondary origin as a pressure effect. 



Its origin is admittedly obscure, but it seems possible that 

 it may be primary, indicating irregular deposition of albite 

 substance during growth. The presence of excess mineralizers 

 may have been effective to this end. 



(5) Vide Flett, J. S. : Mem. Geol. Surv. Eng. and Wales, 

 Geology of Newton Abbot, 1913, p. 60. E. W. Hughes: Geol. 

 Mag., Jan., 1917, p. 18. 



((30 F. Becke: Denk. Kais. Akad. Wien., vol. lxxv., p. 28, 

 1906. 



