148 KEPOKT— 1891. 



just noticed. Near the centre of the lower edge is an ill-formed crystal 

 of nepheline (n), with a small crystal of sanidine closely adjoining. The 

 small cr3'stal marked a, near the upper edge, with a large grain of black 

 iron-ore penetrating into it, is brownish-green augite ; and there are a 

 number of small columnar crystals of this mineral, of fine green colour, 

 scattered throughout the section. The four large black grains — one at 

 the lower edge — represent, no doubt, augite converted into iron-ore, as 

 indicated by the grain above the nepheline crystal having closely attached 

 a particle of green augite, conforming with it in outline. The ground- 

 mass consists of clear allotriomorphic nepheline, densely filled with pale- 

 green microlites (fegirine ?) and colourless ones of sanidine, and contains 

 small grains of iron-ore sparingly scattered through it. 



Fig. 2. — The ground-mass in this is similar to that of Fig. 1. Near the 

 upper edge is shown a large green augite crystal (a), cut at right angles to 

 the vertical axis, but somewhat deformed at the upper and lower corners 

 by two irregular grains of augite breaking into it. A thin black zone of 

 ferruginous matter exists close to, and parallel with, its outlines, and it 

 shows numerous cleavage-cracks parallel to the prism. Included in it are 

 two large and several small grains of magnetite. In convergent polarised 

 light a fine optic axis is seen near the long right-hand edge, which is the 

 trace of the orthopinacoid, while the other outlines represent the traces 

 of the clinopinacoid and prism. There are several smaller augite crystals 

 shown: one (a), near left-hand edge, broken into by a large grain of 

 magnetite ; s, near middle of right-hand edge, is sanidine ; and a large, 

 irregularly-contoured nepheline (») is shown near lower edge ; a small 

 square one, a little to the right, in conjunction with two small prisms of 

 sanidine (s). The small black grains are magnetite, and so is, no doubt, 

 also the large oblong grain, though probably occupying the place of a 

 former augite crystal. 



Fig. 3.— This has been drawn under a greater magnifying power, in 

 order to give an idea of the appearance of a part of the allotriomorphic- 

 nepheline ground-mass, which is tolerably free of microlites. It is seen to 

 be traversed by innumerable fine cracks, which do not, however, conform 

 to the granulation in light blue and dark resulting on rotation of the stage 

 of the microscope between crossed nicols. At the right-hand edge near the 

 top is shown part of a large green augite crystal (a) ; and there are a number 

 of smaller, rather ill-formed crj'stals of this mineral seen in various parts 

 of the section, especially near the lower rim. The black iron-ore grains^ 

 prove, by their isometric outlines, to be magnetite. Some colourless- 

 transparent, long-acicular microlites, seen here and there in the ground- 

 mass, are doubtless apatite, as they show extinction parallel to their length 

 in polarised light. 



Figs. 4 and 5 are drawn in ordinary light from the sides of the dark 

 base of the coarsely-porphyritic rock forming the 1.5ft. dyke on the pro- 

 montory at Portobello. Magnified 2G diams. 



Fig. 4. — This shows, in the centre of upper part, portion of a large 

 particle of water-clear isotropic glass (g), its terminating outlines outside 

 the figured portion being irregular and rounded. With the exception of 

 an included grain of magnetite and a few narrow strings of microlitic 

 inclusions, it is quite clear throughout. Next to it, on the left-hand edge, 

 is part of a large prismatic crystal of rust-brown hornblende (h), quite free 

 from inclusions of iron ore. Below this, on same edge, is part of a small 

 prismatic crystal of green augite (a) ; and right opposite to this, close to 

 right-hand edge, is another crystal of green augite cut somewhat obliquely 

 to the vertical crj-stallograpbic axis, as indicated by its shape and the 

 prismatic cleavage-cracks. Its outlines represent the traces of the ortho- 

 and clino-pinacoids and the prism. The small clear patch between it and 

 the edge consists of isotropic glass, and so does also the small rectangular 

 clear particle [g) below it. At the edge above it, near the large glass- 



