ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 159 



tion of useful cavities to be subsequRntly filled with metalliferous 

 matter.* Sheeted structure iu veins must also be distinguished 

 from combed structure, which is altogether a phenomenon of 

 deposit. In connexion with true sheeted structure there may be 

 traced the following gradation. 



(«). An ordinary fissure vein-filling in a fairly compact 

 rock. Of this condition nearly all the lodes afford examples in 

 some part of their course. 



(b). The same in a "sheeted" country rock. The following 

 examples will serve as illustrations. At Wheal Metal in Breage, 

 the principal (metal) lode runs a few degrees north of east (mag.) 

 and underlies to the northward about two feet in a fathom. The 

 killas is for the most part permeated with tourmaline (tourmaline- 

 schist), and in some places highly silicified — and is almost 

 horizontal — yet in very many places a kind of jointing exists 

 by the side of the lode parallel to it, and therefore directly 

 across the bedding, so forming a succession of false walls. Similar 

 phenomena are common in a great many other veins traversing 

 killas, both of tin and copper. 



At the Ruby Iron Mine near St. Austell, a compact yein of 

 red hematite traverses more or less altered granite, which 

 frequently exhibits this sheeted structure for many feet on each 

 side of the lode. 



(c). A sheeted mineralized belt associated with one or 

 more main veins. 



Pednandrea and Grreat Wheal Fortune afitord examples of 

 this stage. 



(d). A "sheeted" earthy or clayey vein-filling in a 

 "sheeted" country rock. 



Examples of this stage are also common in the tin and 

 copper mines. In many instances it is quite impossible to say 

 where the "lode" ends and the "country" begins, all being 

 sheeted together, although no part of the lode is highly 

 crystalline. 



(e). A " sheeted " mineralized belt not associated with any 

 main vein. 



* The origin of these four forms of tabular jointing has been dealt with 

 sufficiently in Chap. 1, Sec. 2. 



