ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 155 



are many sucli layers, some being less distinctly crystalline than 

 others — or even apparently compact until examined microscopic- 

 ally. Such examples are very common in copper veins in the 

 Gwennap district. Many have been cited by Delabeche and 

 Henwood, and a very remarkable case was figured by me in 

 1873.* 



In lead veins fine examples have occurred at Wheal Rose 

 and Wheal Penrose near Helston, and in iron lodes at Eestormel 

 near Lostv\"ithiel,f and at Pawton near St. Columb.J The 

 evidence afforded by combed structure as to the successive 

 re-openings of fissures, and as to changes in the character of the 

 underground circulation, will be discussed in a future chapter. 



"Vughs"are incompletely filled spaces in fissure lodes, usually 

 lined with well crystallized vein -stones, and commonly associated 

 with combed structure in the vein. Both the lodes described 

 have afforded fine examples ; in the case of Dolcoath an unusually 

 large one was described by Mr. John Eule in 1818.§ 



"Flucan" seems to be partly a result of chemical changes m 

 the lode-filling or in the adjacent country rock, partly a 

 deposit from circulating water, and partly the result of a 

 grinding produced by the motion of the walls of the fissure on 

 each other. It is abundant in certain parts of both the Dolcoath 

 and Perran lodes. 



"Brecciated structure" is comparatively rare at Dolcoath — 

 extremely common at Perran. A fine example of this structure 

 in a lead lode is that at Wheal Mary Ann, described by Dr. 0. 

 Le Neve Poster. || 



"Concretionary structure" is also very common in the Perran 

 lode — notably in the Grreat Retallack portion. Fine examples at 

 Belistian and at New liosewarne are described by Mr. Henwood^ 

 and Dr. Foster.* 



*Proc. Inst. Mech. Eiig., Cornwall Meeting, 1873, pi. 36. 



fSee Moissenet Lodes of Cornwall, p. 85, note. 



X Report Miners Assoc, 1875, p. 26. 



§ Trans. Roy. Geol. Soc. Corn., 1, p. 225. 



II Trans. Roy. Geol. Soc. Corn., ix, p. 155. 



'^Trans. Roy. Geol. Soc. Corn., v. 



*Bep. Miners Assoc, 1866. 



