N 



406 SKY. MINERALS. 



into a friable mass, so as to be mistaken for marie and 

 used as manure. 



It presents scarcely any varieties of crystallization : 

 the predominant, I might almost add the universal form, 

 is that most common one consisting of very flat tetraedral 

 prisms, terminating in tetraedral pyramids, of which the 

 faces are placed on the edges of the prism. These are 

 aggregated in distinct fasciculi, parallel or divergent, of 

 which the groups sometimes affect the form of the consti- 

 tuent crystals. 



In the neighbourhood of Loch Eynort nodules of great 

 size occur, of a variety which is far from common, but 

 which has I believe been found in the Faroe islands. 

 These are either hollow or solid, and sometimes reach 

 the enormous dimension of four and even five feet. The 

 hollow specimens are crystallized within, in the fasciculated 

 forms already described. The peculiarity of this variety 

 consists in its extreme frangibility : the least effort is 

 sufficient to detach the plates of which it is formed, 

 and it therefore falls 4 to pieces in the very act of pro- 

 curing it, unless great care is taken; the fracture is 

 distinguished by an uncommon degree of the pearly 

 lustre so characteristic of this mineral. So great is the 

 frangibility, that the jarring of the hammer at one end of 

 a large nodule is often sufficient to destroy the whole; 

 and it not unfrequently happens that when a large piece 

 is obtained entire and has been laid down, although ap*- 

 pearing uninjured and resisting a strong effort of the 

 hands to break it, yet in a few minutes it falls to pieces 

 with a violence not unlike that which is known to happen 

 in unannealed glass that has received an injury. This 

 variety is sometimes white, and much resembling sper- 

 maceti in aspect, but in the greater number of instances 

 it is of a delicate flesh colour. I may add to this enu- 

 meration that I procured at Talisker a few specimens 

 presenting the primitive form of this mineral. 



