OLDER PEUinOTITKS OF SCOTLAND. 1:501 



the usual brilliant colours, but presents a somewhat curious 

 appearance from the abundant enclosures which it contains, causing 

 it to appear of a dusty grey tint. The numerous fissures which 

 traverse the olivine grains are often stained of a bright yellow colour ; 

 along them are developed numerous cavities which are seen in some 

 cases to be united by a ramifying system of canals, and not un- 

 frequently contain liquids with a moving bubble. In most cases, 

 however, they appear to be filled with brown or black decomposition- 

 products like those in the augite-crystals. Along the same fissures are 

 seen to be developed the curious dendritic stars or networks of a black 

 or brown colour, and these are sometimes present in such numbers as 

 to render the planes of the fissures black and opaque. In addition 

 to these enclosures along the planes of the fissures, other stellar 

 ones make their appearance in prodigious numbers within the sub- 

 stance of the crystal, communicating to it the dusty appearance 

 already described. These are seen to be arranged in. a series of 

 planes parallel to the optic axis of the crystal, and the star-like 

 inclusions are often mingled with and pass into others in the form of 

 thin brown or black plates. 



Among the commonest of the accessory minerals of this rock are 

 felspar, biotite, a ferriferous enstatite (hypersthene), magnetite, 

 and chromite or picotite. 



The felspar is a plagioclase, which in its altered condition 

 offers a striking contrast to the fresh augite and olivine of the 

 rock. It is full of cavities, and its substance is often seen to have 

 undergone more or less complete conversion into various secondary 

 products. 



The biotite, when unaltered, is of a deep brown tint, but is often 

 Schillerized by the development of dark-coloured enclosures in 

 planes parallel to the basal plane. Under these circumstances the 

 substance of the crystals becomes much paler in tint. 



The ferriferous enstatite (hypersthene) exhibits the usual marked 

 pleochroism of that mineral when undecomposed, but it usually 

 shows a great tendency to serpentinous alteration. 



The magnetite and chromite or picotite form only a very subor- 

 dinate part of the rock. The latter mineral by its decomposition 

 appears to communicate a very striking chrome-green tint to 

 portions of the rock. 



Some of the peridotites of the higher mountains in central Rum 

 exhibit a far less perfectly granitic structure, small crystals and 

 granules of olivine being mingled with finely granular augite, as in 

 the porphyro-granulitic dolerites. They resemble such dolerites 

 without their felspar. These rocks have a specific gravity of 3-09, 

 The olivine of these rocks is often more or less darkened by the 

 multitude of black stellar inclusions which it contains. 



These granular peridotites of central Eum are intersected by 

 veins of many interesting rock-varieties. Among these occur 

 porphyritic peridotites consisting of large individuals of olivine 

 scattered through a granular base of olivine and augite and gabbros, 



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