250 AUSTRALASIAN ANTARCTIC EXPEDITION. 



observed in hand-specimen, while herring-bone structure is very characteristic. The 

 alteration associated with the striation is often so arranged as to produce a kind of 

 hour-glass appearance in the sections. A curious feature of these augites is the frequen^ 

 elongation parallel to the c axis, one section parallel to (010) measuring more than 

 6 mm. in length by about -7 mm. Some of the crystals show a marked bending into 

 a circular arc, with undulose extinction. This bending causes a tapering in some of 

 the sections, which, combined with the central pinacoidal twinning-line and the herring- 

 bone structure, produces the appearance of a goose-quill, an effect heightened by the 

 curiously serrated edges of the crystals. This serrated effect is possibly a result of the 

 bending of the crystals during their growth, as suggested by Iddings 1 for the curved 

 felspars of the Obsidian Cliff rhyolite. Much of the augite is graphically intergrown 

 with the plagioclase. 



The iron ore is mostly ilmenite, in characteristic plates and skeletal forms ; there 

 is also much ilmenite and magnetite in tiny granules scattered through the mesostaeis. 



The rock is much stained with brownish-green and brown decomposition products, 

 which penetrate the pyroxene peripherally and along cracks, and also discolour large 

 portions of the mesostasis. (See Plate XXXVIII, Figs. 3, 4, and 5.) 



2. ORDER OP CONSOLIDATION. 



In the finest-grained rock the felspar has very evidently crystallized first, followed 

 by the augite. In No. 732A this is not so evident ; the fabric is subophitic, but the 

 augite is often nearly idiomorphic and only marginally indented by the felspars, as 

 though the former had been the first to start crystallizing and had continued during 

 part of the period of crystallization of the felspar. In No. 733 the graphic intergrowth 

 of felspar and pyroxene indicates much simultaneous crystallization of these two 

 minerals. 



It is noteworthy that much of the iron ore has crystallized late, and that the 

 apatite is largely concentrated in the mesostasis. This is often the case in basic rocks 2 , 

 and is in harmony with the common experience that the last or pegmatitic products 

 of consolidation of basic magmas are often rich in apatite and iron ore. 



Teall, dealing with the Whin Sill dolerite 3 , has remarked on the fact that the 

 coarse-grained phases of that rock exhibit graphic or perthitic intergrowths of their 

 constituents. A somewhat similar statement might be made about these Horn Bluff 

 rocks. 



3. DISCUSSION OF THE PETROGRAPHICAL FEATURES. 



These three rocks are evidently to be placed among the quartz-dolerites, although 

 quartz does not enter very conspicuously into the constitution of the mesostasis. There 

 is an almost complete absence of biotite and hornblende, which are common minor 

 constituents of the typical hunnediabas and kongadiabas. 



1 Iddings : Igneous Rocks, vol. i, p. 226. 



* cf. for example Elsden, Q.J.G.S., vol. Ixiv, 1908, p. 289. 



' Q.J.O.S., vol. xl, 1884, pp. 640-57. 



