ORIGIN AND DEVELOPMENT OF OKB-DEPOSlTS. 137 



Floors. These have relations on the one hand with the 

 impregnations just described, and on the other with the " gash- 

 veins " and " bedded- veins," which are so characteristic of many 

 limestone districts. Tin-floors in granite or other eruptive rock 

 seem to be mere local expansions of the lode passing between 

 approximately horizontal joint-planes for a certain distance. 



In stratified rocks aggregations of ore lying between bed- 

 planes are often known as " bedded-veins " or floors — of course 

 such aggregations coincide in position with the beds, lying 

 horizontal or standing highly inclined as the case may be, though 

 they would only be called floors by the miner when horizontal 

 or nearly so. At Botallack tin-floors occurred in the kill as — 

 at the junction of the killas and granite — and in the granite. 

 In one instance seven successive tin-floors were noticed, separated 

 by beds or layers of ferruginous slate and schorl, and they yielded 

 according to Mr. Hawkins about 1 per cent, of black tin.* 

 Similar floors occurred formerly at Wheal Eeeth and at Wheal 

 Vor.f 



The most interesting examples of this kind of formation 

 of recent working occurred at the Parka Mines in St. Columb, 

 and were described by Dr. Foster in 1874. The lodes here were 

 of very little importance, but certain lateral off-shoots from them 

 lying between the bedding planes of the killas (fig. 7 and 8, Plate 

 III) were extremely rich. If the killas had been here 

 approximately horizontal, these interposed off-shoots would have 

 been called floors by the miners. :j: The average produce of the 

 tin stuff treated was in 1874 more than 5 per cent., and in that 

 year the mine yielded 231 tons of black tin — a remarkable yield 

 from a mine not more than 43 fathoms deep. 



Sec. 5. — Examples of true fissure Lodes. 



{a) General characters of lodes. Lodes are, as Mr. Henwood has 

 very truly observed, "the principal repositories of metals and 



*See description by Hawkins, Trans. Boy. Geol, 8oc. of Corn., vol. 11, p. 31. 

 Came ibid pp. 326-331. Henwood, ibid v, p. 13. note. It is much to be 

 regretted that neither of these authors has given a sketch section. 



f Henwood, ibid, p. 328. 



X Report Miners Association of Cornwall and Devon, 1875. 



