136 THE INCLUSIONS OF THE VOLCANIC ROCKS 



is there perfectly idiomorphic except for the inclusion of stout crystals of magnetite. 

 Apatite and magnetite are abundant in stout crystals, either in the druses or included 

 in any of the above minerals. 



The interstitial glass and ground-mass possess interesting characters. Very often 

 there is a pale brown glass remarkably free from microlites, and in such cases there is 

 a gradual passage from the base of the inclusion into the ground-mass of the trachyte 

 by a gradual increase in the number of felspar microlites. In other cases the ground-mass 

 of the samdinites is more coarsely crystalline than that of the host, from which it differs 

 also in the absence of flow structure (Fig. 5, PI. I). 



The microsanidinites differ considerably from the samdinites in outward appearance, 

 for they are typically compact, fine-grained rocks whose felspathic nature is not always 

 clearly evident at first sight. They are generally light-coloured, but sometimes contain 

 darker, coarser- grained, and miarolitic centres, whilst one specimen (1753) is altogether 

 dark grey and vesicular. 



In section they prove to have a mineralogical composition similar to that of the 

 sanidinites, but they differ notably in structure. They are composed for the most part 

 of small quadrate prismatic felspars of which by far the majority are untwinned, or 

 show only Carlsbad twins and are to be referred to sanidine. The refractive indices are, 

 however, slightly higher than those of the sanidine of the sanidinites and are seldom 

 lower than that of mo nochlor- benzene (/u = l - 525) so that presumably they belong to 

 soda- sanidine. A few of the larger prisms show typical anorthoclase twinning, while 

 occasionally more elongate plagioclase (oligoclase-albite) is also found. Besides these 

 small quadrate felspars there are generally present much larger pseudoporphyritic 

 plates of anorthoclase, which partially envelop the sanidine. A yellow olivine is very 

 abundant in grains of similar size to those of the felspar mosaic, and is in places dis- 

 tinctly ophitic in structure. iEgirine-augite also occurs, but is more idiomorphic. 

 Magnetite in small grains is widespread, whilst apatite is rare. Fig. 6, PI. I, gives a 

 general view of the rock. 



The dark miarolitic centres approach more nearly in structure to the coarse sani- 

 dinites, but are much darker in colour owing to the predominance of tegirine-augite 

 in small prismatic crystals. Miss F. Cohen, of Sydney University, kindly undertook 

 a goniometrical examination of these, the results of which are appended to this paper. 



The microsanidinites appear in most cases to pass insensibly into the ground-mass 

 of the host. The dark variety (1753) forms an exception, as it is sharply delimited 

 from the trachyte by a thin zone of large anorthoclase and olivine crystals. 



A fragment of rock showing holocrystalline structure, specimen (1146), found as an 

 erratic at Cape Royds, may possibly be an inclusion from a trachyte. It is a mottled 

 yellowish grey and black rock of medium to fine grain, which bears some resemblance to 

 the sanidinites in one corner, where it is more felspathic and more coarsely crystalline. 

 It consists of anorthoclase prisms arranged in orthophyric fashion, with abundance 

 of interstitial aegirine-augite and olivine largely replaced by magnetite, with a smaller 

 amount of barkevicitic hornblende in lypically allotriomorphic forms. Apatite and 

 magnetite are richly present. The rock is thus a basic type of alkaline trachyte or 

 orthophyre. Whether it was brought to the surface as an inclusion or is a fragment 

 of a dyke remains uncertain. 



Sanidinites and Microtinites in the Kenytes 



Five felspathic inclusions have been obtained in the kenytes; with them will be 

 described two others found as erratics at Cape Royds on account of their similarity. 

 The mineralogical composition of these rocks is shown in the following table : 



