246 AUSTRALASIAN ANTARCTIC EXPEDITION. 



I. THE DOLERITES OF THE HORN BLUFF. 

 1. PETROGRAPHY. 



The rocks present certain variations in texture, but consist essentially of the 

 same minerals, plagioclase, pyroxene, and iron ore, with a mesoetasis of micropegmatite. 

 It will be convenient to preface short descriptions of each of these rocks with an account 

 of the characteristics of the principal mineral constituents. 



Plagioclase is as a rule fresh, but occasionally showing slight alteration to sericite. 

 The habit is usually stout prismatic, but in the finest-grained rock (No. 732B) the 

 prisms are more elongated. Zoning is marked, especially in the two coarse rocks 

 (Nos. 732A and 733), and twinning is present on the albite and occasionally on the 

 Carlsbad law, pericline twinning being visible only in the felspars of the coarsest phase 

 (No. 733). The limits of chemical composition are bytownite (Ab 20 An 80 ) to andesine 

 (Ab 55 An 45 ) in the case of No. 732A, and Ab 29 An 71 to Ab 45 An 45 in No. 733. The felspar 

 of No. 732B shows less strong zoning and is about Ab 35 An 65 . 



The pyroxene is almost entirely monoclinic. It is very pale-grey in colour, 

 sometimes changing peripherally to green or brownish, and without noticeable pleo- 

 chroism. Some of it is apparently ordinary augite or diopside, but much is of the 

 magnesium-rich variety, known as enstatite-augite or magnesium-diopside, characterised 

 by an optic axial angle which is variable, but always much smaller than that of ordinary 

 augite, approaching in the limit. Extinction angles up to 43 have been obse.ved, 

 but it seems as though extinction and double refraction both decline with the optic 

 axial angle. This is well seen in No. 732A, where two pyroxene individuals often occur 

 in parallel intergrowth, that with the smaller optic axial angle being distinguished 

 by a slightly lighter colour in ordinary light. 



In addition to the usual prismatic cleavage, there is often in both pyroxenes an 

 extremely fine basal or salite striation, which, when combined with simple or repeated 

 twinning parallel to (100), gives rise to herring-bone structure. This basal striation 

 is not at all constant in its occurrence, often not extending all the way across a crystal, 

 and being developed in irregular patches. It is often brought into prominence by 

 local yellowish alteration of the pyroxene, with decline in the birefringence. For the 

 most part, nothing definite was observed tending to show that this basal parting 

 betokens twinning, as has been sometimes suggested, (cf. Iddings, Rock Minerals, 1906, 

 p. 305), although in one case there was something that looked like simultaneous ex- 

 tinction of alternate lamellae. 



A similar very fine striation is also sometimes observed parallel to (100), and 

 this is likewise often incomplete, and accompanied by alteration. With the develop- 

 ment of these salite and diallagic striations, there is often local obliteration of the ordinary 

 prismatic cleavage. 



Twinning, when present, is usually of the normal type, parallel to (100), but 

 occasional cruciform twins on (101) are observed, 



