104 



place to an iridescent tarnish after the mineral has been exposed to 

 the air for any length of time. Besides the cleavage parallel to the 

 general lamination of the mineral, two sets of subordinate cleavage 

 planes cut it in directions perpendicular to the first, and intersect 

 each other in such a manner as to cut the laminae into rhombs. 



This is an important source of the Bismuth of commerce, but 

 with us it occurs only in very small quantities, and is then usually 

 associated with Telluric Bismuth, Bismuthine, Bismuth Ochre, 

 Wolfram, Scheelite, and other interesting mineral species. 



The last element on the list is Graphite, Plumbago, or Black 

 Lead. This was formerly be considered as more or less of a Car- 

 buret of Iron ; but it is now usual to regard the Iron, or the other 

 matter associated with the Graphite as adventitious, and to class 

 Graphite as one of the two allotropic mineral forms of Carbon. 

 The mode of occurrence and the method of working the Plumbago 

 found in the renowned mines of Borrowdale have been so fully 

 described by my colleague, the late Mr. Ward, in the Geological 

 Survey Memoir on the Northern Part of the Lake District ; and 

 again by Mr. Postlethwaite of Keswick in his essay on " Mines 

 and Mining in the Lake District," that it would be out of place for 

 me to repeat the substance of their remarks here. Plumbago is 

 said to occur at Bannerdale Mine on Saddleback, as well as in the 

 better-known locality of Borrowdale. 



IL 



The next division comprises the compounds of the Metallic 

 Elements (i) with the Elements of the Arsenic Group — the 

 Arsenoids, viz., Bismuth, Antimony, and Arsenic; (it.) with the 

 Elements of the Sulphur Group — the Thionids, viz., Tellurium, 

 Selenium, and Sulphur; and, (m.) with the Elements of both the 

 Arsenic and the Sulphur groups. 



(/.) Amongst the minerals recorded by Mr. Ward as occurring 

 in the Lake District mention is made of Nickel, which is stated 

 to have occurred in the form of specks in a quartz-vein in High 

 Snab Bank, north of Robinson. Nickel has not yet been known 



