276 ProceediTigs of the Royal Physical Society. 



weathering with a rusty brown crust. The contrast between 

 the mode of weathering of the serpentine and the gneissose 

 rocks would strike the most casual observer. The Heog 

 ridge presents a bare sterile appearance, the scanty nature of 

 the vegetation being due to the character of the rock. It is 

 now generally admitted by petrologists that serpentine is a 

 truly metamorphic rock, which has been developed by the 

 alteration of some pre-existing mass. It is a hydrated silicate 

 of magnesia of a fine grained texture, and is usually associated 

 with steatite, carbonate of lime, diallage, and other substances. 

 Near the Loch of Cliff:', and along the north side of the Heog 

 Hills, I observed several instances of the talcose schists 

 gradually assuming a serpentinous character, and eventually 

 merging into serpentine. It is of importance to note that 

 talc becomes very abundant in places near the edge of the 

 serpentine area as at Norwich Bay. Like the well-known 

 Portsoy serpentine, that of Unst varies in colour from a 

 green to a reddish brown. This variation in colour has been 

 attributed to " the proportion and degree of the oxidation of 

 the iron which it contains." The serpentine of Unst has long 

 been famous for the chromate of iron which it contains, which 

 was first discovered by Dr Hibbert, to whose zealous labours 

 we are likewise indebted for the first description of the 

 geological structure of the Shetland Isles. This ore has been 

 worked in the Heog Hills and near Harolds wick Bay. The 

 chromate of iron is disseminated through the rock, and in 

 places it becomes so abundant as to constitute an important 

 ingredient in it. Nevertheless, this ore occurs after the manner 

 of a vein, the great masses being in the form of what Cornish 

 miners call "bunches," but the name conveys the impression 

 of amplifications on an elsewhere thinner mass. In Unst the 

 vein, as actual chromate of iron, cannot be seen to pass from 

 one " bunch " to the other, but the two sides of the sheathing 

 mineral can be traced. Near Buness also the vein throws off 

 at least one continuous offset, which, maintaining a diverse 

 course, must be held to indicate the vein nature of the larger 

 mass. 



To the south-east of the great mass of serpentine occurs the 

 diallage rock of the Ford Hill. 



