OLBEll PEETDOTITES OF SCOTLAND. 415 



Fig. 6. Crystal of olivine, with magnetite developed along the cracks and in- 

 vading the substance of the crystal. In addition stellate enclosures 

 of magnetite &c. are making their appearance in negative crystals 

 arranged in a series of parallel planes traversing the crystal. From 

 the troctolite at the top of Halival, Isle of Eum. Crystal shown 

 magnified 100 diameters. (See page 383.) 



7. Portion of a crystal of olivine in which magnetite has been developed 



along the cracks to such an extent as to render black and opaque 

 nearly the whole crystal. Portions of the olivine-substance, partially 

 converted into serpentine, remain here and there in the midst of the 

 mass. From the olivine-gabbro, Beinn More, Isle of Mull. Shown 

 magnified 50 diameters. [In the same rock many of the olivine- 

 crysta,ls are seen rendered altogether black and opaque by the de- 

 velopment of magnetite particles in them.] (See page 383.) 



8. Crystal of biotite cut at right angles to the basal plane, and showing 



Schillerization along the planes coinciding with those of the principal 

 cleavage ; from the scyelite of Loch Scye, Caithness. Seen as mag- 

 nified 50 diameters. (See pages 383 and 405.) 



9. Two thin flakes of the same biotite, lying parallel to the plane of easy 



cleavage of the mineral, magnified 100 diameters. The enclosures are 

 seen to be more or less regular plates, very similar to those found in 

 the hypersthene of Labrador. Two grains of magnetite are also seen 

 enclosed in the same crystal. (See pages 383 and 405.) 



Plate XIII. 



Varieties of the Ultra-basic Eocks of Scotland. All the sections are repre- 

 sented as seen with a magnifying-power of 30 diameters 



Fig. 1. Olivine rock (dunite) of the Shiant Isles. Consisting of a mass of 

 granules of clear olivine, only rarely showing faint signs of serpenti- 

 nization, A few scattered particles of brown augite, of anorthite, 

 and of magnetite (or chromite) are scattered among the olivine-grains 

 which make up the bulk of the rock. The oUvine is clear and almost 

 entirely free from enclosures of secondary origin. In the form of 

 the olivine-grains this rock resembles the dunite of St. Stephan in 

 Upper Styria ; but in the perfect freshness of the olivine it finds its 

 analogue in the typical dunite of the Dun Mountain, near Nelson, 

 New Zealand. (See page 394.) 



2. Porphyritic olivine rock (dunite) from the flanks of the mountain of 



Scuir na Gilean in the Isle of Eum. The rock consists of an aggre- 

 gate of minute olivine-grains with a little augite, through which larger 

 crystals of olivine are scattered. These larger crystals exhibit the 

 dusty appearance produced bj'' the development of numerous stellate 

 and tabular enclosures in negative crystals, and lying in two inter- 

 secting sets of planes within the crystal. The olivine is quite free 

 from any trace of serpentinization. (See page 391.) 



3. Olivine-augite-enstatite rock (Iherzolite) from the top of Halival, in 



the Isle of Eum. The structure of this rock is intermediate between 

 the granitic and the ophitic. The olivine forms rounded grains con- 

 taining a few large stellate enclosures, and is often enclosed in the 

 augite or enstatite ; the augite is bright green in colour, and is not 

 improbably a chrome-diopside, and the enstatite. is a ferriferous one 

 (proto-bronzite or proto-hypersthene), of a rich brown colour, with 

 very marked pleochroism. Scattered through the rock are a few 

 grains of chromite or picotite. Felspar is only present as a rare and 

 accessory ingredient of the rock. The augite and the enstatite show 

 slight traces of Schillerization. These two minerals cannot be distin- 

 guished in the drawing. (See page 392.) 



4. Ophitic picrite (augite-olivine-rock) from the Shiant Isles. The rock is 



made up of very lai-ge crystals of deep brown augite, which enclose 

 numerous rounded grains of olivine, while these latter, in turn, 

 enclose rounded grains of chromite or picotite. The minerals of this 



