OF THE ROSS ARCHIPELAGO 143 



A brown glass is found somewhat abundantly in certain bands, both as inclusions 

 in the felspar and augite and as an interstitial base. In the latter case it encloses 

 well-shaped octahedra of magnetite and is crowded with arborescent crystallites, 

 In addition there is occasionally a clear mineral of moderate refringence and low bire- 

 fringence, which is biaxial with a moderate axial angle and optically negative. In 

 crushed fragments of the rock this mineral is seen to be well crystallised and 

 appears to be orthorhombic, with pinacoids, basal plane, and dome faces. It has not 

 been definitely determined. 



If the presence of glass be neglected, the rock has the mineralogical composition 

 of an augite syenite (of the soda syenite family) and may be termed an orbicular augite 

 syenite. The glass included in the minerals might have arisen by remelting, and in one 

 of the specimens has very much the appearance of having arisen in this way ; that 

 occurring interstitially, however, appears to be the result of a normal stoppage of 

 crystallisation due to sudden cooling. The alkaline affinities of the rock ally it to the 

 volcanic series of Ross Island, and the hypothesis is put forward that it was torn from 

 its place before consolidation was complete, and brought to the surface either in a lava 

 or a volcanic breccia. 



V. PLAGIOCLASE-PYROXENE INCLUSIONS IN THE TRACHYTE 



OF MOUNT CIS 



Among the fine-grained compact inclusions from Mount Cis there are four charac- 

 terised by a mottled greenish grey appearance. These prove on microscopic 

 examination to consist of a basic plagioclase and pyroxene with subordinate magnetite 

 and sphene, and in one case calcite. The plagioclase generally occurs in large tabular 

 plates with both Carlsbad and albite twins, and is referable from extinction angles 

 to labradorite. The pyroxene in three of the specimens (T 7, P 33, and 1969) is a pale 

 green monoclinic variety with oblique extinctions up to 44° and a high axial angle. 

 In the other (E 35) it is an almost colourless variety with straight extinctions and is 

 optically negative and therefore referable to bronzite. The pyroxenes generally have 

 an ophitic relation to the felspars, but in parts of the rocks the structural relations 

 have been more or less broken down, and the pyroxene is found as smaU'grains inter- 

 mixed with and included in small felspar plates (Fig. 1, PI. III).* In places a 

 brown glass appears, either as inclusions in the felspars, or as an interstitial matrix in 

 which small euhedral felspars are embedded (Fig. 2, PI. III). In one rock (E 35) the 

 felspar is accompanied by a colourless, more strongly refringent mineral which appears 

 to be wollastonite. In the same rock there is a vesicular glassy selvage against the 

 trachyte, in which large plates of anorthoclase are developed. 



These inclusions must be interpreted as fragments of dolerites that have been par- 

 tially melted and recrystallised, with a slight admixture of trachytic material on the 

 edges. The dolerites appear to have no alkaline affinities, and can scarcely be derived 

 from the trachytic magma or its allies. | They are probably fragments of sills intrusive 

 into the Beacon sandstones such as occur on the mainland. 



* From the resemblance of these apparently grannlitic parts of the rock to some of the pyroxene 

 granulites that are found as erratics at Cape Royds, the writer was at first inclined to regard these 

 rocks as inclusions of granulites, and this view was published in the preliminary account of the 

 Geology by Professor David and Mr. Priestley, Compt. Rend. Congr. Geol. Inter. Stockholm, 1910. 

 The absence of the characteristic clove-brown wedges of sphene and the ophitic character of the 

 least disturbed parts of the rock are against this view. 



"j" They present little resemblance to the gabbroid nodules described above. 



