Cn. XXXVIIL] CHEMICAL DEPOSITS IN" VEINS. 627 



has often been an intimate connection between metalliferous veins and 

 hot springs holding mineral matter in solution, yet we must not on 

 that account expect that the contents of hot springs and mineral veins 

 would be identical. On the contrary, M. E. de Beaumont has judi- 

 ciously observed that we ought to find in veins those substances, which, 

 being least soluble, are not discharged by hot springs, — or that class of 

 simple and compound bodies which the thermal waters ascending from 

 below, would first precipitate on the walls of a fissure, as soon as their 

 temperature began slightly to diminish. The higher they mount 

 towards the surface, the more will they cool, till they acquire the ave- 

 rage temperature of springs, being in that case chiefly charged with the 

 most soluble substances, such as the alkalis, soda, and potash. These 

 are not met with in veins, although they enter so largely into the compo- 

 sition of granitic rocks* 



To a certain extent, therefore, the arrangement and distribution of 

 metallic matter in veins may be referred to ordinary chemical action, 

 or to those variations in temporature, which waters holding the ores in 

 solution must undergo, as they rise upwards from great depths in the 

 earth. But there are other phenomena which do not admit of the same 

 simple explanation. Thus, for example, in Derbyshire, veins containing 

 ores of lead, zinc, and copper, but chiefly lead, traverse alternate beds 

 of limestone and greenstone. The ore is plentiful where the walls of 

 the rent consist of limestone, but is reduced to a mere string when they 

 are formed of greenstone, or " toad-stone," as it is called provincially. 

 Not that the original fissure is narrower where the greenstone occurs, 

 but because more of the space is there filled with veinstones, and the 

 waters at such points have not parted so freely with their metallic 

 contents. 



"Lodes in Cornwall," says Mr. Robert W. Fox, "are very much 

 influenced in their metallic riches by the nature of the rock which they 

 traverse, and they often change in this respect very suddenly, in passing 

 from one rock to another. Thus many lodes which yield abundance 

 of ore in granite, are unproductive in clay-slate, or killas, and vice versa. 

 The same observation applies to killas and the granitic porphyry called 

 elvan. Sometimes, in the same continuous vein, the granite will contain 

 copper, and the killas tin, or vice versa?\ Mr. Fox, after ascertaining 

 the existence at present of electric currents in some of the metalliferous 

 veins in Cornwall, has speculated on the probability of the same cause 

 having acted originally on the sulphurets and muriates of copper, tin, 

 iron, and zinc, dissolved in the hot water of fissures, so as to deter- 

 mine the peculiar mode of their distribution. After instituting experi- 

 ments on this subject, he even endeavored to account for the preva- 

 lence of an east and west direction in the principal Cornish lodes by 

 their position at right angles to the earth's magnetism ; but Mr. Hen- 

 wood and other experienced miners have pointed out objections to 



* Bulletin, iv. p. 12V 8. f R. TV. Fox on Mineral Veins, p. 10. 



