2 Ladi/ Rachel McRobert — A NepJieline-Si/enite Boulder. 



The specimen is fairly coarse in texture, distinctly heterogeneous, 

 and is composed of two parts separated by a perfectly distinct line — 

 a dark-grey portion witli good needles of hornblende and abundant 

 sphene, traversed by lighter veins composed mainly of felspar, 

 nepheline, and sodalite, and a light-grey part in wliicli ferro-magnesian 

 minerals are quite subordinate, wliile twinned crystals of orthoclase 

 are conspicuous and with a marked tendency to orientation along the 

 line of separation. Under the microscope the rock shows a fairly 

 coarse hypidiomoi'phic structure. The minerals are all very fresh, 

 and there is practically no sign of incipient decomposition along the 

 cleavage or margins. In the pale variety of the rock the minerals 

 are the same as in th.e darker portion, and only vary in an increased 

 proportion of felspar and nepbeline, and in their orientation. These 

 circumstances and the fact that the small pale veins traversing the 

 rest of the rock are of similar type seem to indicate that this is the 

 last consolidated portion of the magma. 



The minerals in their order of importance are — 



Potash-felspar, mainly perthitic orthoclase in good twin crystals, 

 best developed in the pale variety ; also in close association with 

 nepheline, often enclosing it poecilitically. 



Soda-felspar, albite .showing its characteristic fine lamellar twinning, 

 present in very small amount and more idiomorphic than the orthoclase ; 

 it occurs also in very fine perthitic intergrowtli with orthoclase. 

 Microcline is absent. 



Nepheline in perfectly* clear large plates or intergrown with felspar, 

 often crowded with fluid inclusions and small grains of augite. 



Sodalite, crowded with innumerable liquid inclusions and minute 

 crystals of the basic minerals, and presenting a cloudy appearance, 

 with occasional aggregate polarization. It tills up spaces left by tlie 

 felspar and nepheline, and is allotriomorphic. Rounded patches are 

 also enclosed by large plates of felspar, so that it must have started 

 cr3'stallizing simultaneously with the felspar and continued sub- 

 sequently. It is pale blue in hand-specimen. 



Hornhlende , greenish brown and very pleochroic ; « = X= yellow or 

 pale brown, ^=7"= dark brown, 6' = /^= dark brown; very intense in 

 good idiomorphic crystals, with an extinction angle of 17° or 18°. The 

 crystals are grouped together iu close association with the iron-ores, 

 sphene, and aegirine-augite. It is the chief ferro-magnesian con- 

 stituent, and is moulded around the other above-named constituents. 

 With segirine-augite it occurs in parallel intergrowth, and often 

 forms the outer shell of crj'stals in which the kernel is the pyroxene. 



JEgirine-augite, a pale-green variety and occasionally lilac, slightly 

 pleochroic from lemon yellow to pale green, with an extinction angle 

 of about 25° or 30° and quite subordinate to hornblende. 



Sphene, in excellent double-wedged lozenges frequently twinned, of 

 a curious pale yellow grey, with a pleochroism marked by variation of 

 intensity rather than of colour. The margins are often corroded. 

 Its inchisions are of apatite, felspar, nepheline, and even sodalite in 

 some instances, thus further confirming the long period of crystal- 

 lization of tlie latter. 



Magnetite, in irregular grains and hexagonal crystals, containing 



