108 ' MESSES. J. E. DAXYNS AND J. J. H. TEALL ON THE 



2. COLOTJE AND TeXTUEE. 



The texture of the rocks is granitic. Colour varies from black in 

 the peridotites and serpentines to nearly white in the thin veins of 

 eurite. The general colour of any intermediate rock is of course 

 determined by the relative abundance of the ferro-magnesian and 

 felspathic constituents. The grain of the rocks is in most cases 

 sufficiently coarse to enable the individual constituents to be easily 

 recognized. The orthoclase of the granites is of a pale flesh colour. 

 The granite with large crystals of orthoclase is the only rock which 

 is distinctly porphyritic ; for the tonalites, diorites, and peridotites 

 are composed of individuals having fairly uniform dimensions. A 

 pseudo-porphyritic aspect is given to some of the peridotites by 

 the serpentinization of the olivine. 



3. MiNEEALOGICAL COMPOSITION. 



The following minerals enter into the composition of these 

 rocks : — 



(a) Essential Minerals. Iron ore, olivine, enstatite, augite, 



diallage, hornblende, biotite, plagioclase, orthoclase, micro- 

 cline, quartz. 



(b) Primary Accessories. Sphene, apatite, zircon. 



(c) Secondary Accessories. Serpentine, epidote, chlorite, mica. 



(a) Iron Ore. — In the ultra-basic rocks the iron ore occurs mainly 

 in the anastomosing strings which traverse the olivine. It has no 

 direct connexion with the process of serpentinization, for it is 

 found alike in the perfectly fresh and in the altered mineral. The 

 distribution of these veins in the serpentinized rocks is a valuable 

 criterion for the estimation of the amount of olivine originally 

 present. Apart from the veins in the olivine, we find iron ore 

 occurring as allotriomorphic grains and more rarely as crystals 

 (octahedra). In one or other of these forms it is present in all 

 rocks, but it is, of course, far more abundant in the basic than in 

 the acid rocks. In the latter it can only be regarded as an un- 

 important accessory. 



The general absence of titanic acid, or its presence only in very 

 minute quantities, has been established by chemical analysis, so 

 that the iron ore must be non-titaniferous. 



Iron ore may occur as inclusions in any of the other constituents, 

 and it is the only mineral present in these rocks of which this 

 can be said. 



Olivine. — This mineral is present in the form of colourless, allotrio- 

 morphic grains, generally more or less serpentinized, but in a few 

 cases perfectly fresh. The serpentinization of olivine has been so 

 frequently described that no detailed account of it is necessary on 

 the present occasion. It is of a perfectly normal character. The 

 anastomosing veins of magnetite have already been described. In 

 addition to these we find occasionally in the centre of an unaltered 



