Vol. 57.] MOUNT GIENAE, JU^'AGAE^. 43 



same age as the diorite ; among these is the rock described in the 

 following pages. It occurs on the margin of a nepheJine-sj^enite 

 containing very little ferromagnesian material.^ In hand-specimens 

 it is fine-grained, and deep black except for numerous white specks. 



On examination under the microscope, the small white spots are 

 seen to be colourless spaces, which are as a rule accurately circular 

 and evidently sections of spheres. In most cases they are free 

 from any of the darker constituents of the rock, though now and 

 then a small crystal of hornblende is visible, or a long prism of 

 the same mineral projects into the colourless area.^ The mineral 

 composition of these spaces differs from point to point, and will be 

 dealt with later. 



Outside these colourless areas are abundant crystals of a slightly 

 greenish-brown hornblende; some of the darker crystals, that 

 have a slightly reddish tint, resemble the soda-hornblende barke- 

 vikite. They are of all dimensions, from minute microlites up 

 to 1 millimetre or more in length. Common green hornblende is 

 occasionally found. 



A pale-green non-pleochroic augite occurs in much less quantit)' 

 than the hornblende, and is the only important constituent that 

 shows a likeness to any of the minerals of the diorite. It is 

 sometimes found in comparatively large crystals, approaching 1 

 millimetre in length. These show occasionally terminal patches of 

 dark-green pleochroic aegirine-augite in crystalline continuity.^ 

 In other cases it forms the nucleus of a brown hornblende, likewise 

 in crystalline continuity.^ The augite also occurs in small grains 

 cither disseminated through the rock or, more frequently, collected 

 in glomeroporphyritic masses, usually irregular in outline, but in 

 some cases showing definite rectilinear contours with angles that 

 suggest pseudomorphs after hornblende. 



Biotite is very rare, only one or two occurrences being noted 

 in a large number of microsections. Sphene is fairly common. 



A few small granular crystals of an apparently uniaxial mineral 

 with a high refractive index, but very low double-refraction, were 

 seen in one of the irregularl3'-shaped colourless areas. It may 

 possibly belong to the eudialyte-eucolite group. It is of a pale 

 yellow colour, like some of the eudialytes from Kangerdluardsuk 

 (Western Greenland) ; but, so far as could be ascertained by the 

 examination of these small grains in convergent light, the sign was 

 negative, as in eucolite, not positive as in eudialyte. 



The interspaces between the coloured constituents are filled with 

 colourless material, which will now be described in conj unction 

 with that filling the circular areas. 



^ Other portions of this rock are composed almost entirely of small grains of 

 nepheline, which occasionally show the parallelism of flow-structure. 



■^ Occasionally similar spaces with irregular boundaries are seen. These are 

 sometimes due to the coalescence of two spheres : not infrequently they contain 

 coloured minerals. 



^ The angle C:c appears to measure about 60°. 



* In one case a twinned augite was seen to be surrounded by a twinned horn- 

 blende having the same plane of composition. 



