ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 119 



the red sandstone conglomerates resting on the lode at West 

 Doddington in W. Somerset is perfectly conclusive ; here the lode- 

 fissure itself may have been the channel through which the 

 cupriferous solutions were introduced subsequent to the consoli- 

 dation of the rock. The copper in the sandstones consists of 

 impregnations and concretions of blue and green carbonate — 

 similar to those of the Alderley Edge sandstones in Cheshire ; 

 it was formerly worked on a considerable scale.* 



Sec 3. — Examples of Stochworks. 



In many mining regions, bands or belts of " country-rock " 

 are found which are traversed by numerous thin veins — or their 

 numerous joints are thinly lined — or they are sprinkled through- 

 out with small spots of metalliferous substance ; the whole mass 

 being thus rendered of considerable value. An ore-mass of this 

 character is called by the Germans — from whom we have derived 

 many of our mining terms — a "stock," and a working upon 

 such a mass a " stock-werk" or — as the term has been adopted 

 in England — a " stock-work." 



Since the individual strings or nests of mineral are usually 

 insignificant, it is necessary in stock- work mining to remove the 

 whole mass of impregnated rock and to treat at any rate the 

 greater portion in order to concentrate and separate its valuable 

 contents. As the ores so distributed are often very small in 

 quantity compared with the whole mass of the rock, e.g. with 

 copper ores 1 per cent, or less and with tin ores from 3 to 10 lbs. 

 of tin oxide to the ton, a concurrence of favourable circumstances 

 is necessary to enable them to be worked with profit - such as 

 cheap labour, land of little agricultural value on which to 

 deposit the refuse, a good outlet for the said refuse so as to keep 

 the workings clear, a good supply of water for concentration 

 purposes, and, if possible, water-power for crushing, a body of 

 the impregnated rock so large as to permit of working on a 

 considerable scale, a genial climate allowing work to be carried 

 on without serious interruption, &c. Even with all these advan- 

 tages many are so poor that they remain unworked, and very few 

 will pay to work except as open quarries. 



* Delabeche, Report, &c., 609, and Leonard Horner, Trans. Geol. Soc, Lond., 

 Ill, pp. 352, 363. 



