90 HORN EXPEniTION — PETROLOGY. 



magnetite. A little chlorite, a few flakes of biotite, and a little iron pyrites are 

 present as accessory minerals. Granular .augite occurs in ophitic patches, enclosing 

 sometimes two or three felspar crystals. The order of crystallisation appears to 

 have been as follows : — magnetite, felspar, olivine, and augite. The rock occurs 

 intruding gneiss near the head of Ellery Creek. 



Olivine Dolerite. Nos. 195 and 179. Plate II., Fig. 1, and Plate IV., 

 Fig. 4. Sp. gr. 3-07. 



The felspar of this rock resembles in all respects that described in specimen 

 No. 99. The augite has its usual characters, the only feature that need be noticed 

 is its occurrence in large ophitic masses, well shown in Plate IV., Fig. 4. The 

 olivine, which is present in large quantity, is almost entirely undecomposed ; there 

 are numerous cracks, however, in the crystals, which are filled with tine magnetite 

 grains. A few grains of hypersthene are present with comparatively feeble 

 pleochroism, c being green, and d pink. This rock occurs between Slip-panel Gap 

 and Ellery Creek, as a dyke-intruding gneiss. 



Fine-grained Doierite. Nos. 79 and 1-21. Plate II., Fig. 2. Sp. gr.. 

 No. 79, 2-95; No. 121, 2-97. 



The plagioclase crystals are lathe-shaped and have high extinction angles. 

 The augite is the ordinary brown basaltic variety occurring in fairly uniform 

 grains, some of which are quite fresh, while others ai'e altered into pseudomorphs 

 of chlorite, fibrous hornblende, and probably also .serpentine. Ophitic structure is 

 exhibited in parts. This rock intrudes gneiss between Slip-panel Gap and Ellery 

 Creek. 



Dolerite. No. G4. Plate IV., Fig. 3. 



This rock, which is much decomposed, consists essentially of plagioclase, 

 augite and ilmenite. The plagioclase is much decomposed with the production 

 probably of a moderate amount of saussurite. The augite has crystallised out 

 early, there being no inclusions of felspar in it. Simple, and perhaps also 

 multiple, twinning is exhibited by some of the augite crystals. Alteration has 

 gone on in places, granular epidote, chlorite, and a fibrous hornblende being 

 developed. The place of magnetite is taken by ilmenite, Plate IV., Fig. 3, 

 which occurs in the form of irregular flat grains, exhibiting the characteristic 

 hatched appearance in incident light. A little secondary quartz and a few 



