OF MOUNT EREBUS, ANTARCTICA 99 



placed by Rosenbusch * in the Trachydolerite group. On the other hand, the presence 

 of basalt hornblende, kaersuetite and high titania indicate affinities with the Kulaites. 

 Though so closely allied in chemical composition the Erebus pumice and the Observa- 

 tion Hill trachytes are very different in appearance, the former being a hemicrystalline 

 kenyte with huge anorthoclase phenocrysts, the latter an even-grained holocrystalline 

 trachyte in which only hornblende occurs as phenocrysts. The Observation Hill 

 trachytes are apparently kenytes in chemical composition, cf. Table I, B and E (p. 98) 

 with Table II, A, B, and C (p. 110). 



The Mount Cis trachyte, J. 13, is in mineral composition and microscopic structure 

 similar to the phonolitic trachytes from Cape Adare and Cape Crozier, of which analyses 

 are given for comparison. The first of these was described by David, Smeeth, and 

 Schofield (Proc. Roy. Soc. of N.S.W., 1895), and the latter by Prior {op. cit.). This 

 rock type is also akin to the phonolitic trachytes of Dunedin, New Zealand, described 

 by Marshall (Q.J.G.S., vol. lxii, 1906), and the trachyandesites of the Warrumbungle 

 Mountains. It will also be seen that Washington's Cumal Phlegrose is an almost 

 identical rock. 



The Mount Cis trachyte is in the American system of classification a monzonose, 

 in which class the Warrumbungle trachyandesites also fall, but the margin between 

 these rocks and the phlegroses is very slight. High titania and the presence of some 

 zirconia are characteristic features of these rocks. 



J. 1 (1911) is in hand-specimens a dark basalt-like rock, which nevertheless has 

 a silky sheen that discloses its trachytic nature. Under the microscope it is seen to be 

 porphyritic- hiatal in phaneric first generation phenocrysts and perpatic. The ground- 

 mass is holomicrocrystalline and varies in fabric from true trachytic near the vesicles 

 and inclusions to pilotaxitic away from them. 



The sparing phenocrysts in this slide consist of green aegirine-augite and small 

 nephelines, which in this rock type antedate the felspar. The nephelines are idiomorphs, 

 and possess a low double refraction, a hexagonal cleavage and refractive index of 1*54. 

 It appears that this rock prior to its extrusion has had many nepheline idiomorphs, most 

 of which have changed to aggregates of socialite and analcite. These masses preserve 

 the outlines of nepheline, and hence are regarded as pseudomorphous after that mineral. 



The ground-mass consists of lath-shaped felspar microlites with the properties of 

 anorthoclase, and eegirite or acmite (both with straight extinction), which occur in 

 hypidiomorphic prisms and allotriomorphic aggregates behaving ophitically to felspar. 

 Magnetite occurs as an abundant minor constituent in allotriomorphic grains ; brownish 

 glass occurs as a minor interstitial substance ; zircon needles occur sparingly in minute 

 needles in the felspars and may be looked upon as an early consolidation product. 



In addition there are a few grains of deep brownish-green arfvedsonite amphibole 

 enveloping deep blue riebeckite grains. The latter are pleochroic from deep purp fish- 

 black to dark brownish-green in elongated sections, and has a cleavage of 55^°. 

 Brown cossyrite-like hornblende seems also to be present in minute amount. Sometimes 

 this occurs as a nucleus of a3girine. Its pleochroism ranges from bluish-green to 

 greenish-brown in sections at right angles to the length of the grains. 



The rock as described must be regarded as a phonolitic trachyte allied to Cumal 

 Phlegrose (see Washington, TJie Roman Comagmatic Region, p. 27). 



Plate I, figs. 1 and 2. 



J. 13 (1922). This rock resembles J. 1 both in hand-specimen and under the 



microscope. It contains the same felspars together with eegirine-augite, aegirine, 



*jGesteinlehre, zweite Auflage, p. 355. 



