86 Transactions of the 



a prominent row of hills. Its breadth varies from one to two miles, 

 and its junction with the serpentine is well seen at the head of the 

 small inlet on the south, near Belmont ; in the north it unites with 

 the mica slate of Saxaford and Burra Fiord, Its strata take a westerly 

 dip, at an angle of nearly 45°, and the direction of its hills is parallel 

 to its line of strike. It assumes the common form of gneiss, the fel- 

 spar predominating, and the quartz often occurring in distinct veins. 

 In some of the sections on the west shore it displays remarkable con- 

 tortions and curvatures. In the neighbourhood of Woodwick and 

 along the western side of Burra Fiord, it undergoes a striking change, 

 which is, probably, only another stage of metamorphism. 



Here, owing to its silvery aspect and fracture, it might almost be 

 called mica schist, but the quantity of felspar it contains forbids the 

 appellation. This micaceous gneiss differs from the other variety in 

 its colour, which is reddish brown, and in its softer texture. It is 

 interesting from the great number of garnets, or crystals of grenatite, 

 scattered throughout it. These are most abundant at Woodwick. 

 The crystals are dodecahedral in shape, and vary in size from a pin's 

 head to half an inch in diameter. They are best seen on the 

 weathered surface of the rock, for, being hai-der than the matrix, they 

 stand out in relief. They occur again at the head of the Loch of 

 CliflF, and along its north-western side. These garnets are opaque 

 and red in colour. This micaceous rock is, probably, only a gradation 

 between the true gneiss and the mica schist of Saxaford, but I have 

 thought it better to indicate it by a different colour on the map. It 

 is not fomid elsewhere in the island, and it extends from Woodwick up 

 to and along the west coast of Burra Fiord. Two striking dykes can 

 be seen in this fiord. They run parallel on opposite sides of the inlet, 

 in a S.W. direction, both issuing from the Cliff of Saxaford. Another 

 dyke is also seen near the head of Haroldswick Bay, and indications 

 of intrusive masses in the peaks of Heog's Hill. 



The Serpentine, in a horizontal section, shews the largest area, 

 which is pear-shaped in outline. It is separated from the gneiss in 

 the south by a thin strip of mica and talc slate, which becomes wider 

 in the north. The eastern boundary of the serpentine is a line 

 running N.E. and S.W., and at Gallow Hill this rock is only half a 

 mile broad, but expands like a wedge towards Balta. Its external or 

 weathered surface is of a brownish yellow colour, and the interior of a 

 dark brown mottled with black. Masses are often found of an apple- 

 green colour, which pass into " noble serpentine." This latter is of a 

 fine green colour, semitranslucent, compact, and hard, and takes a good 

 polish. The serpentine rock contains much magnesia, its softness and 

 l^erhaps its solubility are due to this mineral, besides silex, alumina, and 

 oxidulated iron. It forms a beautiful white sand, which seems noxious 

 to animal life, for Balta Sound is comparatively destitute of raarinfe 

 fauna. The rock is also insalubrious to vegetation, as the bare and 

 rocky hills indicate. The sea, as I have remarked, has made larger 

 bays into the serpentine than in any other of the formations. 



The serpentine contains the valuable chrome ironstone (FeO Gr^ O3), 



