142 



THE INCLUSIONS OF THE VOLCANIC ROCKS 



IV. ORBICULAR AUGITE-SYENITE : PROBABLY AN INCLUSION 



An Erratic from half-way between Cape Royds and Cape Barne, near Mount Cis. 

 The reasons for assuming that this rock is an inclusion of some volcanic rock are, first, 

 the alkaline affinities of the rock, and secondly, the abundance in it of a brown glass 

 similar to that described in so many of the above inclusions. 



The hand- specimens (1974 and E33) are small angular fragments which show for 

 the most part a banded character, and only in one case a complete spherule. The 

 accompanying sketch (Fig. 2) of a polished surface, kindly drawn by Mr. W. N. Benson, 



of Sydney University, gives a clear 

 idea of the macrostructure. The 

 dark bands and the centre of the 

 spherule owe their colour in part to an 

 augite, but more particularly to mag- 

 netite. The clear bands consist pre- 

 dominantly of felspar. 



The augite is a brownish green 

 variety without marked pleochroism. 

 The dispersion of the bisectrices is so 

 strong that the mineral often does not 

 completely extinguish, but the extinc- 

 tion angle Z A c is approximately 32°. 

 The axial angle is nearly 90°. From 

 the brownish colour and strong dis- 

 persion the mineral must be classed 

 with the titaniferous augites. It occurs 

 in anhedral elongate plates, which are 

 relatively large when they are found in 

 the centres of the spherules or between 

 the dark bands, and in these cases are 

 set more or less radially. Occasionally 

 in the small clear bands it occurs in small rounded grains. In the dark bands it is 

 generally in much smaller crystals and elongated tangentially to the spherules. In 

 both cases it is rich in magnetite inclusions, arranged perpendicularly to the elonga- 

 tion, and occasionally encloses a brown glass. 



The felspars show a great diversity of twinning ; untwinned forms are abundant, 

 and so are forms with cross-hatchings of various degrees of sharpness from mere 

 moire extinctions to twinning scarcely to be distinguished from microcline twinning ; 

 simple albite twinning with low extinction angles is comparatively rare. The 

 refractive indices of all these lies not far from that of Canada balsam. More exact 

 determinations in liquids of known indices show that none lie below 1525, and most 

 lie between L536 and 1-540. A plagioclase in cleavage flakes shows one index below 

 L540 and one above; it therefore belongs to oligoclase; the rest of the felspar 

 appears to belong to anorthoclase with unusually high refractive indices, and probably 

 therefore rich in the anorthite molecule. Like the augite, the felspars vary in size 

 according to their position. Within or near the dark bands they form a fine-grained 

 polygonal mosaic ; between the dark bands and in the centre of the spherules they 

 have a tendency to an elongate radial habit. They enclose small magnetite grains 

 and an abundance of fine colourless needles, too small for exact determination. Some 

 larger prisms with cross-fractures are certainly apatite, but the majority of the 

 needles are probably sillimanite. 



Fig. 2 



