CU/ion College Scientific Society. 87 



which nms throughout it in veins and pockets. This mineral has a 

 crystalhue appearance ; the mineral map shews the area oyer which 

 it extends. The ore is divided into three qualities, according to the 

 amount of serpentine with which each is mixed. The first quality, 

 now all but exhausted, is visibly free from the containing rock, and is 

 of a coal-black colour, opaque, and ^-ery heavy, and has a submetallic 

 lustre. It is subject to disintegration, is brittle, and its fracture is 

 uneven. The second quality has a mottled look, while the third 

 consists merely of specks of ore, disseminated throughout the ser- 

 pentine, resembling grains of gunpowder. The chrome ironstone, 

 which has proved highly remunerative to the proprietors, was dis- 

 covered by Dr. Hibbert, in 1817, but it was not worked till 1822. 

 It first fetched the price of £10 per ton, but soon fell, as ore of a 

 better quality can be obtained from Russia and America ; however, 

 mani;facturers find it convenient to mix the t(vo. It is used for pro- 

 ducing yellow and green pigments, and the colours magenta and sol- 

 ferino, in oil, porcelain, and water-colour painting. Part of its 

 protoxide of iron is replaced by magnesia (derived from the 

 serpentine), and part of the oxide of chromium by alumina. Alone, 

 it is infusible before the blowpipe flame, and it fuses with difficulty iu 

 borax ; but the emerald green colour of oxide of chromium becomes 

 very apparent when the bead is heated in the inner flame. 



The few quarries for this ore are now no longer worked. They are 

 situated chiefly at Swinauess, Haroldswick, and Balta (Burness House). 

 A little of it is also said to occur in the Island of Fetlar. 



At the point called Swenee Xess, north of Balta, that rare mineral, 

 native hydrate of magnesia, or Brucite (MgH), was discovered by Dr. 

 Hibbei't, many years ago, former mineralogists believing it to be a 

 white talc. It occurs in thin veins through the serpentine, iu 

 aggregated, foliated plates, of silvery white colour ; it is translucent 

 when newly fractured, but, on exposure to the air, it absorbs carbonic 

 acid and becomes white. Its composition shews a large percentage 

 of water, — 



Magnesia .... G8.97 

 Water .... 31.03 



100.00 



An analysis of it, by Thomson, has the following result,- 

 Maguesia .... 66.67 

 Protoxide of Maugauese . 1.57 



Protoxide of Iron . . 1.18 



Lime 0.19 



Water 30.39 



100.00 



Swenee Ness is the only British locality for this mineral, and even 

 here it is now very scarce, and an impure kind only is to be procured, 

 which is mostly composed of carbonate of lime. I will mention the 

 other minerals of Unst hereafter. 



