MINERAL VEINS. 579 



The mining districts of Cornwall and Devon ^ are grouped around the granitic 

 masses, and Mr. J. A. Phillips considers that they for the most part originated 

 subsequently to the consolidations of the elvan-courses. He observes that both 

 the sedimentary and igneous rocks of Cornwall are traversed by innumerable 

 mineral veins, which, although principally composed of siliceous materials, contain 

 ores of tin, copper, lead, and various other metals. Veins yielding ores of tin and 

 copper have usually a direction approximating to east and west, and are seldom 

 found at any considerable distance from the junction of the granite and killas, 

 particularly if elvan-courses do not occur in the neighbourhood. These veins, 

 which are called " lodes," are intersected, nearly at right angles, by others known 

 as " cross-veins " (or " cross-courses "), wliich sometimes yield lead or iron ores, 

 but are otherwise, excepting in the immediate vicinity of lodes, usually un- 

 productive.- 



The metallic worth of lodes is liable to much variation, while 

 their mode of occurrence is subject to uncertainty, owing to the 

 * throws,' ' shifts,' and ' heaves ' (faults) which affect the strata in 

 which they occur. Surface indications are seldom considered very 

 trustworthy evidences for mineral wealth at a depth, and it some- 

 times happens that the capel (or cab) is more valuable than the 

 'leader' or lode itself. Alluvial deposits, however, often furnish 

 good evidence of the ores contained in the rocks of a district. 



Dr. C. Le Neve Foster has shown that near Redruth and Camborne the lodes 

 are not always fissures containing mineral matter, but may be tabular masses of 

 rock, altered and rendered metalliferous. Indeed, he is disposed to believe that at 

 least half the tin-ore of Cornwall is obtained from such masses of stanniferous 

 altered granite. These lodes maybe termed "Tabular stockworks." The name 

 "Stockwork" (from the German Stock-iverk) is usually applied to large masses of 

 rock impregnated with metallic ores, or intersected by a number of mineral veins 

 at short distances apart, sometimes crossing one another in all directions. It 

 probably owes its origin to the method of working formerly often adopted for such 

 deposits, which were wrought by chambers arranged in tiers or stories.'^ Dr. 

 Foster recommends the following definition: — "A mineral vein or lode is a 

 tabular mineral mass formed, more or less entirely, subsequently to the enclosing 

 rocks."* In Cornwall the term Huel (or Wheal) signifies a mine-work. 



Much of the material forming mineral veins is no doubt due 

 to ordinary aqueous deposition, or to segregation from the 

 neighbouring rocks, but in regard to the formation of the metallic 

 minerals much uncertainty exists; sea-water, as before mentioned, 

 contains traces of various metals, and thermal waters are known 

 to convey the soluble salts of some of them (see p. 537) ; but both 

 chemical and electrical action may have been instrumental in pro- 

 ducing our mineral veins. 



The majority of 'precious stones' have been formed in rocks 

 that have been subjected to intrusions of eruptive matter ; but no 

 ' gems ' of any importance are found in England or Wales.* 



1 See J. H. Collins, Handbook to the Mineralogy of Cornwall and Devon, 

 1871 ; W. J. Henwood, Trans. R. G. S. Cornwall, v. ; De la Beche, Report on 

 Geol. Cornwall, etc., 1839. 



2 Q. J. xxxi. 320. 



^ Q. J. xxxiv. 652, 654, 658. 



* G. Mag. 1884, p. 513. 



* See Judd, Volcanoes, ed. 2, p. 146 ; Morris, Gems and Precious Stones, Pop. 

 Sc. Rev. vii. 123, 1868. 



