﻿Rev. T. G. Bonney — The LherzoUte of the AriSge. 63 



approach to a regular crystal outline ; the enstatite usually irregular 

 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 irregular in size. Surface finely granular, 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 irregular 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 



granular or slightly silky texture. The cleavage parallel to goPod 

 is generally well exhibited, though not so close as a rule as in 



diallage ; a more interrupted cleavage parallel to coP is also some- 

 times fairly distinct, as in Rosenbusch, 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 till 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 be 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 microlitbs. 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, rich 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 develoj)ed. 



The picotite occurs in very irregular 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 



