248 Proceedings of the Royal Physical Society. 



large enough to attract notice with low powers of 30 to 50 

 diameters. In occasional eddies of the fluxion structure, or in 

 interspaces between the crystals, the glass appears with a 

 IJ-inch objective to be somewhat dusty, the dust grains being 

 drawn out into the fluxion streaks, accumulated in little centres 

 between the currents, or massed into clot-like patches. With 

 a higher power these portions are seen to consist of crowds 

 of minute, pale-yellowish, clear, drop-like granules, distributed 

 with considerable uniformity. Applying a magnifying power 

 of 800 diameters, we find that while most of these granules 

 are spherical, not a few are elongated and dichotomous at 

 one or both ends (see fig. 5, PL V.). Whatever be their 

 form they are invariably inert in polarised light. The largest 

 do not exceed on an average about ^o^oo of ^^ i^^h in diameter, 

 the ordinary spherical globulites averaging perhaps about 

 •lowoo- I^ the dusty areas where these globulites are most 

 developed they are not only to be observed in patches and 

 streaks through the glass, but are specially noticeable crowded 

 upon the pale microliths to be immediately described. 



2. Opaque Grains and Eods. — Eound the augite and mag- 

 netite crystals some parts of the glass are rendered dark by 

 a quantity of black dust formed of minute, black, opaque 

 particles, and short, straight fibres. This black border is 

 usually separated from the enclosed crystals by a very narrow 

 interspace of clear glass. Its component grains and straight 

 rods may be magnetite. They are often to be noticed round 

 isolated magnetite crystals, and sometimes arranged in chief 

 mass at two opposite ends of a crystal as if by a kind of 

 magnetic polarity. They also encrust translucent microliths. 



3. Microliths. — These are of two varieties — {a) Pale- 

 yellowish, clear, straight dart-shaped rods, ranging from less 

 than YoVo- ^P to fully -j-^ of an inch in length. In some cases 

 I noticed them curved, and even spiral. They may be observed 

 in all parts of the glass, but chiefly round the crystals and 

 crystalline granular patches of augite. Solitary prisms of this 

 mineral occur in the glass, bristling all round with an arma- 

 ture of these fine spines, and reminding the observer of some 

 radiolarian and foraminiferal forms of life (fig. 5, PI. VI.). 

 These microliths, so constantly and abundantly divergent 



