Rn\ T. G. Bonnet/ — The Lherzolite of the Ariege. 
63 
approach to a regulär crystal outline ; the enstatite usually irregulär 
and longish ; the olivine appears to have crystallized the first, but 
I think the difference has not been great. It generally forms about 
| of the whole mass of the rock. The picotite, from its shape, seems 
to have crystallized last. 
The olivine occurs in more or less rounded, transparent, colourless 
grains, very iivegular in size. Surface finely granulär, something 
like frosted glass. Colours with crossed Nicols often very beautiful, 
commonest from a translucent greenish yellow to a yellowish green, 
and from a bright to a purplish pink. Owing to the peculiar texture, 
one of these tints often overspreads the other something like a shot 
silk. The mineral shows the usual rather irregulär cracks, indi- 
cating its imperfect cleavage. These often cause, by imperfect 
cohesion, colour bands, which are also common near the edges of 
the grains. Not seldom we find in the olivine small vermicular 
cavities arranged in slightly wavy bands. These appear to be 
sometimes empty, sometimes filled with a brownish mineral, perhaps 
iron peroxide. They lie in some cases in the planes of imperfect 
cohesion, and then have often a dendritic character. There are 
occasional clots of an opaque dust-like mineral, probably magnetite, 
and thin fibrous brown films, strongly dichroic, which may either be 
mere stains or minute plates of iron-glance. The last are often 
associated with the picotite. 
The enstatite is transparent, colourless in ordinary light, with a finely 
granulär or slightly silky texture. The cleavage parallel to 00 P 00 
is generally well exhibited, though not so close as a rule as in 
diallage ; a more interrupted cleavage parallel to ooP is also some- 
times fairly distinct, as in Rosen husch, Mikroscop. Physiogr. Tab. 
viii. 44. In cases where the specimens have a less characteristic 
aspect, I have found the principal cleavage planes better exhibited 
by rotating the microscope stage tili the plane of the principal cleavage 
is nearly parallel to the plane of Vibration of one of the crossed Nicols, 
when, as the crystal approaches its darkest aspect, the fine cleavage 
becomes more clearly visible. This method (proposed by Tschermak) 
of distinguishing the orthorhombic enstatite from the monoclinic 
diallage will he found very useful in examining Lherzolite. The 
crystals show sometimes wavy bands Crossing roughly at right 
angles the lines of the principal cleavage, formed apparently by 
minute elongated cavities and microliths. Colours with polarized 
light pale yellowish or greyish to various blues. 
The diopside is not generally in well-formed crystals ; it is pellucid 
in the thin slices, and sometimes still retains a faint tinge of green. 
With polarized light, the colours are less diaphanous in aspect than 
those of the olivine, rieh yellowish-brown and puce tints being 
common. The surface is rather variable, but generally moderately 
rough-looking, with often a slightly “ stepped ” aspect. The 
characteristic cleavage, as in augite, is commonly well developed. 
The picotite occurs in very irregulär grains or groups of grains, or 
even films, often looking as if a point armed with a sticky fluid 
had been drawn for a short distance along the slice. Surface rather 
