96 HORN EXPRDITION — PETROLOGY. 



Plate TV. 



Fig. 1(7. — A Lathe-shaped Felspar. No. 25 x •^^. Drawn in ordinary light, 

 but with .some of the twinning planes shown. Linear inclusions of 

 magnetite and augite grains, also fine microscopic dust. 



„ 1/'. — A Felspar Grain. No. 25 x -:!". Drawn in polarised light to 

 show arrangement of minute inclusions. Black lines represent 

 extinguished twin lamellae. 



,, 2i7. — Specimen No. 220 x -4°. Drawn "under crossed nicols." A small 

 grain to show micro-pegmatitic structure ; felspar is extinguished. 



„ 1h. — Specimen No. 215 x -:]°. Drawn " under crossed nicols." A highly 

 magnified grain of micro-pegmatite showing parallel arrangement of 

 quartz and felspar. One of the constituents is extinguished. A few 

 inclusions are shown. 



„ 3. — Specimen No. 64 x '-} . Drawn in reflected light. Shows chaiac- 

 teristic structure of ilmenite. Three sets of dark lines crossing at 60°. 

 The black patches represent iron pyrites. 



„ 4. — Specimen No. 195 x •*..''. Drawn in ordinary light to show ophitic 

 structu re. 



„ 5^. — Specimen No. 25 x '.;'". Drawn in ordinary light. A porphyritic 

 crystal of augite with metallic inclusions surrounded by grains of horn- 

 blende, which pass almost imperceptibly into the augite. 



„ 5/'. — Specimen No. 19 x y. Drawn in ordinary light to show passage 

 of augite into hornblende. 



^^ 6<r — Specimen No. 19 x "./'. Drawn "under crossed nicols" to show 

 bending of twin lamellae of plagioclase. 



,, ^b. — Specimen No. 99 x ".f . Drawn in ordinai-y light to sbow irregular 

 cracking of olivine crystal and infilling of magnetite. 



