138 THE INCLUSIONS OF THE VOLCANIC ROCKS 



clase and andesine. In their interstices another felspar may be detected in small lath- 

 shaped prisms. From its refractive indices between those of monochlor- benzene 

 (/x = 1525) and clove oil (m = l'53ti) it is determined as anorthoclase. It is accompanied 

 by large crystals of olivine and titaniferous augite enclosing apatite, small apparently 

 shapeless plates of haematite and biotite and much magnetite in large and small 

 grains. 



The two erratics (1155 and 1710) are presumed to be inclusions from the fact that 

 they bear a general resemblance to the sanidinites and differ from plutonic rocks in con- 

 taining a considerable amount of clear brown glass. They are further presumed to 

 be inclusions from the kenytes from the abundance of magnetite that they contain, and 

 in the case of (1710) from the presence of biotite. They have not been studied in section. 



Other Sanidinites 



Microsanidinite in a Basic Hock from Rut Point (1238). The host is a reddish brown, 

 somewhat earthy basic rock, and the inclusion a small friable white rock. The powder 

 of the latter examined in oils shows that it is composed almost entirely of anorthoclase, 

 with zircon as the only other constituent in numerous stout prisms. The grain size and 

 general appearance recall the clearer microsanidinites of Mount Cis. 



Erratic Sanidinite, Cape Royds (1357). This is a large angular rock which bears in 

 some places considerable superficial resemblance to the sanidinites, but is not miarolitic 

 like them, and is distinguished by the occurrence of large lustrous patches of black glass. 

 Microscopic study suggests that it is a sanidinite that has been partly remelted. The most 

 abundant mineral is a clear sanidine, partly in large plates with numerous enclosures of 

 glass and leucite, and partly as a fine mosaic. An augite of peculiar yellowish colour is 

 fairly abundant, also partly in large plates full of the same inclusions and partly as small 

 rounded grains clustered together. It has a colour more nearly yellow than green, is very 

 feebly pleochroic, gives extinction angles of 45°, and has strong dispersion of the bisec- 

 trices with an axial angle of nearly 90°. Perhaps the yellow colour is due to the reheating 

 which the rock appears to have undergone. Apatite is also an abundant constituent 

 in stout prisms with sharp pyramidal terminations where it lies in glass, a feature which 

 suggests a late crystallisation for the mineral. T he glass, brown in thin section, occupies 

 relatively large areas of the rock. It has a refractive index slightly lower than that of 

 monochlor- benzene (,u= 1-525). Rosenbusch gives the indices of artificial syenite glass 

 as 1 "520, and of monzonite glass as 1550.* Douglas f obtained results still higher 

 for syenites and even more acid rocks. The refractive index of the brown glass in the 

 sanidinites of Mount Cis is also less than 1525, so that the glass of the rock under 

 description agrees well with what would be expected from the remelting of a 

 sanidinite. 



Within the glass there are two minerals besides the apatite that have clearly separated 

 from the glass on cooling (Fig. 5, PI. II). One is magnetite in quadrate sections of large 

 and small octahedra, in whose immediate vicinity the glass is decolorised. The abund- 

 ance of the magnetite is probably connected with the melting of olivine or biotite, 

 although why the former did not recrystallise is difficult to understand. The other 

 mineral is still more abundant in small sharp colourless perfectly isotropic crystals with 

 low relief. They have sometimes quadrate forms, occasionally truncated at the corners, 

 and sometimes hexagonal forms. A few cleavage traces are observed on both forms, 



* Mikr. Phys., i, ii, Tabelle I, 1905. 



f Douglas, J. A., " Changes of Physical Constants in Minerals and Igneous Rocks, on the 

 Passage from the Crystalline to the Glassy State," Q.J.G.S., lxiii, 1907, p. 153. 



