PLATE v. 

 FIG. r. 



PICRITE. 



HALIVAL. ISLE OF RUM. 



Magnified 27 diameters. Ordinary light. 



The minerals represented are olivine (1), green augite (7), and magnetite. 



The olivine occurs in the form of rounded grains. It contains numerous minute 

 opaque inclusions, which produce a cloudy aspect in certain places. The irregular 

 cracks which traverse the grains are sometimes marked out by a brown staining. 



The green augite forms, as it were, the ground mass in which the olivine grains 

 are embedded, and possesses no definite external form. The portions in the upper part 

 of the figure extinguish simultaneously, and are thus seen to be parts of one and the 

 same crystalline individual. These portions show two sets of cleavage cracks, which 

 intersect approximately at right angles, and give in convergent light an optic axis. 

 The section is, therefore, taken nearly at right angles to the vertical axis. 



Magnetite occurs in the form of crystals, and more or less rounded grains. 



Other portions of the same slide contain olivine grains embedded in large plates of 

 felspar (see plate III., fig. 2), and the latter mineral is somewhat too abundant to make 

 the rock a true picrite. 



This rock is one of those referred to by Professor JUDD (Q.J.G.S., vol. XLL, 

 p. 389). 



FIG. II. SCYELITE. ACHAVARASDALE MOOR, CAITHNESS. 

 Magnified 10 diameters. Ordinary light. 



This figure shows hornblende (8), mica (9), serpentinous pseudomorphs after 

 olivine (6), and magnetite. 



The hornblende occurs usually in large crystalline plates, interrupted by the 

 serpentinous pseudomorphs. The colour is very slight (a almost colourless ; /3 and 7 

 pale green). In the left hand portion of the figure aggregates of imperfectly developed 

 hornblende crystals may be observed. In other portions of the figure, transitions from 

 green to colourless hornblende with the usual rise in the order of the tint under crossed 

 nicols may be observed. 



The biotite also occurs in large plates. Sections at right angles to the basal plane 

 show the characteristic cleavage and change in tint when rotated over the polariser, from 

 a pale to a somewhat darker brown ; but the dichroism is not strongly marked. 

 Cleavage flakes of this mica, when examined with convergent light, give the interference 

 figure of a mineral which is practically uniaxial. Minute tabular or rod-like inclusions 

 of magnetite, arranged parallel to the planes of easy cleavage, occur in certain portions 

 of this mineral; as, for example, in the lower part of the figure to the right. 



The serpentinous substance is grass green in colour, and more or less cloudy, in 

 consequence of the presence of minute opaque granules, tables or rods. The rod-like 

 inclusions are often arranged in parallel planes. In some cases the structure of the 

 serpentinous substance suggests that it is the result of the alteration of enstatite rather 

 than olivine. Magnetite is scattered throughout the slide, mostly in the form of opaque 

 granules and irregular patches. 



This rock has been described by Professor HEDDLE (M.M., vol. V., 1884, p. 260,) 

 and by Professor JUDD (Q.J.G.S., vol. XLL, p. 401). It bears a close relation with 

 the hornblende pricrite of Schriesheim, the principal difference being that the mica is 

 much more conspicuous in the scyelite. 



