154 ORIGIlSr AND DEVELOPMENT OF OEE-DEPOSITS. 



"Gozzans" occur in connexion with a majority of the main 

 lodes in the killas, and in some of the lodes in granite. The 

 finest examples known have been on the "backs" of copper 

 lodes, extending in some instances — as at the United Mines in 

 Gwennap, and Wheal Friendship near Tavistock — to a depth of 90 

 fathoms or even more, if the permanent water-level of the 

 country is so far beneath the surface. Many of the rarer forms 

 of phosphate and arseniate of copper, and an immense number 

 of other rare minerals, have only been found in the gozzany 

 parts of copper lodes. The changes of iron pyrites and of 

 carbonate of iron into gozzan are obvious in innumerable 

 instances. The manner in which these changes have been 

 brought about will be considered hereafter in a future section. 



"Pseudomorphs" of one mineral in the form of another have 

 been frequent in both the lodes while shallow. Such pseudo- 

 morphs are very rare in the deeper parts of the Dolcoath lode, 

 though they have often been found at depths of 200 fathoms and 

 upwards in East Pool, the Grwennap mines. South Oaradon, &c. 

 Still, speaking generally, it may be said that they are far rarer in 

 deep than in shallow mines. 



"Capels" are the silicified walls of the fissures, they occur 

 more especially in tin mines and often indeed contain enough 

 tin to pay for working, but they are known also in lodes yielding 

 copper, lead, zinc, and iron. Besides those characteristic of the 

 Dolcoath lode, very fine examples occur at Wheal Uny and 

 other mines on the great flat lode.* Their peculiarities will 

 be described in some detail hereafter, when the method of their 

 formation comes to be discussed. 



The "combed structure" frequently exhibited by the lead, 

 copper, zinc, and iron lodes of the West of England, and to a less 

 extent by the tin lodes, is a development of successive plates of 

 vein-stone, — or in the rich parts of mixed ore and vein-stone, — 

 the latter being usually highly crystalline. In its simplest form 

 this structure consists of a fissure lined with crystals on each side, 

 having their bases on the walls and their apices directed towards 

 the centre. In some cases the fissure is thus altogether filled up 

 with two sets of crystals meeting in the centre — in others there 



*See Foster. " Great Flat Lode," loc. cit. 



