21 



of the albit-e and anorthite molecules included in the niicro- 



cline would have the effect of altering the percentages to: — 



Microcline ... ... ... ... 33*1 



Plagioclase (Ab,J 23'2 



These figures are obviously at variance with the Rosiwal 

 result, and the composition of the plagioclase is less likely 

 than that obtained by assuming Gartrell's figures for the 

 microcline. 



The rock is a strongly potassic biotite granite approaching 

 aplite. 



(4) ALBITE MICA SYENITE. 



Although this rock does not outcrop anywhere else than 

 at Rosetta Head, the relative proportions of the main con- 

 stituents vary considerably from place to place, and there are 

 likewise textural variations. Sometimes the rock is porphy- 

 ritic and at other, times phenocrysts are absent. For the most 

 part mica is subordinate, but near the contact with the 

 schists it increases in amount until it predominates over the 

 felspar, and in places the syenite actually appears to merge 

 gradually into the country rock. 



Microscopically the typical rock is coarse grained and 

 porphyritic and composed predominantly of albite. Sections 

 parallel to (010) give extinctions of 19° from the basal cleav- 

 age, indicating a practically pure soda-felspar, and the pheno- 

 crysts and ground-mass felspars are apparently of identical 

 composition. Veiy perfect twinning after albite and pericline 

 laws is shown, in addition to simple twinning of very irregular 

 type. A good deal. of the felspar is of the variety known as 

 chequer albite ; in this the twin lamellae instead of being 

 continuous are interrupted, the general effect being a kind of 

 chequer pattern between crossed nicols when the lamellae are 

 broad and the interruptions frequent, but often approaching 

 the grating or spindle structure of the microcline twinning 

 when the lamellae are long and thin. Chequer albite has been 

 found by Flett^i^' and Hughes ^^'^^ in soda rhyolites or quartz 

 keratophyres, and the present writer has found it in similar 

 rocks from Currabubula, New South Wales, but there does 

 not seem to be any record of its occurrence' in rocks similar 

 to those now being described. ^^^^ The chequer structure is 

 apparently peculiar to the albite of very sodic rocks. In the 

 present instance the structure appears to be due in part to a 



(13) Geol. Surv. of Great Britain, Mem. "Geology of Newton 

 Abbot," 1913, p. 60. 



(14) Geol. Mag., 1917, p. 18. 



(15) Except, of course, in the albitites of Cape Willoughby 

 described by Tilley in Trans. Roy. Soc. S. Austr., 1919. 



