On Tridymite-Quurtztrachyte, and on Olivinegabbro. 29 
definition of the crossed twins of tridymite naturally becomes 
very complicated and cannot be pursued at large in this place, 
Fig. 2. 
Line of maximum extinction. Line of maximum extinction. 
SOxe 
‘ydo jo 
our[d 
Plane 
of opt 
axes 
The quartz appears in the thin slices of the rock in very clear 
colourless sections, of a roundish or hexagonal form, often irregu- 
lar by the intruding paste. The glassy interspaces often show 
di-hexaédrical forms, with a fixed bubble; the glassy matter 
filled with needle-like crystallites of a lght brownish colour. 
Fluid cavities seem to be entirely absent. Several quartz sections 
are surrounded with a very fine granular border only definable 
under high magnifying powers as an aggregation of minute grains 
of epidote. 
The biotite appears only very rarely as flakes in the slices, with 
its distinct pleochroism giving light-yellow, brown, and dark- 
brown colours. Magnetite, often in regular forms of octahédrons, 
is distributed through the paste, in some places partly decom- 
posed into a brown oxide of iron, whence the yellow colour of the 
paste. 
Epidote is distributed through the slices abundantly. It ap- 
pears in some as well-formed little crystals, with definable faces. 
One crystal has distinctly the form combined by the faces, 
m (100), t (001), r (101), z (011), n (111). But mostly it occurs in 
the form of grains and little bands, easily discernable by their 
light yellow colour and very vivacious chromatic polarisation. 
The dense aggregations of little grains, which appear as muddy 
clouds in many parts of the paste, are for the most part epidote. 
This mineral seems to be everywhere of a secondary orgin. 
Crystals of apatite are not very numerous; its long prisms are 
sometimes broken, and the single members joined in a pearl- 
like string. 
