776 CHEMICAL DEPOSITS IN VEINS. [Ch. XXXVin. 



and others. I may add that, if veins have been filled with gaseous 

 emanations from masses of melted matter, slowly cooling in the sub- 

 terranean regions, the contraction of such masses as they pass from a 

 plastic to a solid state would, according to the experiments of Deville 

 on granite (a rock which may be taken as a standard), produce a re- 

 duction in volume amounting to 10 per cent. The slow crystalliza- 

 tion, therefore, of such plutonic rocks supplies us with a force not only 

 capable of rending open the incumbent rocks by causing a failure of 

 support, but also of giving rise to faults whenever one portion of the 

 earth's crust subsides slowly while another contiguous to it happens to 

 rest on a different foundation, so as to remain unmoved. 



Although we are led to infer, from the foregoing reasoning, that 

 there 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 min- 

 eral veins would be identical. On the contrary, M. E. de Beaumont 

 has judiciously observed that we ought to find in veins those sub- 

 stances 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 average 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 composition 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 temperature 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 re- 

 duced 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, 



* Bulletin, iv. p. 1278. 



