324 Soiitlioni Cross. 



tlu' Itiii^u prismatic felspars show very fine alhite twin striatinns : 

 syninietrioal extinctions in the twin laniella3 were as low as 

 Kf : ill cleavage flakes the extinction on r (001) was about 5°, and 

 on /* (01(t) about 13^: the specific gravity was between quartz and 

 labradorite from Labrador (about 2-67). Most of the felspar there- 

 fore consists of a rather acid labradorite, but some patches of felspar 

 also occur without twin-striations and moulding the labradorite : 

 these have a refractive index (as determined by the Becke method) 

 lower than that of the labradorite, and may consist of orthoclase, or 

 possibly anorthoclase. The ferro-magnesian patches consist of fairly 

 large irregular grains of a pale yellow olivine, a pale green diopside- 

 like augite with high angles of extinction, and plates of biotite, with 

 some apatite and ilmeuite. (See PI. LIII., fig. 1.) 



This rock is here doubtfully referred to the essexites ; it has some 

 affinities with the augite - syenites, such as the olivine-bearing 

 laurdalite of Brogger. In the absence of data as to its mode of occur- 

 rence, it is only possible to suggest that it might very well represent 

 the plutonic magma, from which were derived both the hornblende- 

 basalts anil also the olivine-bearing anorthoclase rocks or kenytes 

 described later. 



QUARTZ-FELSITES. 



The felsitic rocks found as boulders at Cape Adare probably 

 occur in association with the granites. They are mostly pale yellow 

 halleflinta-like rocks, but some show small phenocrysts of quartz, 

 felspar and muscovite. Other specimens are more coarsely-grained. 

 One of these is a red porphyritic granophyre, showing under the 

 microscope large hexagonal sections of quartz (sometimes much 

 corroded and penetrated by the groundmass) and kaolinised felspar 

 (orthoclase and oligoclase) in a holocrystalline groundmass of inter- 

 locking quartz and felspar grains with micropegmatic structure 

 in jiarts. 



Another coarse-grained specimen from Geikie Land is a pale 

 greenish-gray rock speckled with small phenocrysts of quartz and 

 f(dspar. Under the microscope it shows large and plentiful pheno- 

 crysts of corroded quartz and orthoclase in a holocrystalline ground- 

 ma.ss, consisting of a mosaic of quartz and felspar. Scattered through 

 the slide both in ]»henocrysts and groundmass are shreds of a 

 micaceous mineral and chlorite, similar to those in the quartz grits 

 to be described later: epidote grains have also been developed at 

 the expense of the felspars. 



