328 
quartz solutions in the associated vein did not completely 
infill the cavities. 
V. Tue NATURE AND Composition oF THE WHITE PEGMATITE. 
he 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. ; 
(a) Albitite.—D 5, —2622:— Under the microscope this 
rock is seen to consist essentially of albite. Accessories are 
apatite, in hexagona 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 allotriomorphie. A curious mottled twinning 
shows up in some ions. This has been described as 
“‘chequer albite.’’ 9 In other sections only well-defined albite 
lamellae are present. : : 
A very small quantity of interstitial quartz is present 1n 
the slide. mr 
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.(9 Flett and sa 
have noted its development. in phenocrysts of volcanic rocks 
associated with albite of the usual kind. 
Dx _ © Vide Flett, J. S.; Mem. Geol. Surv. Eng. and Wales, 
~ 7 Geology of Newton Abbot, 1913, p. 60. E. W. Hughes: Geol 
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vol Ixxv., p. 29 
