CHEMICAL PEPOSITS IN VEINS. 
613 
with the ruins of fractured rocks must always have been 
filled with water; and almost every vein has probably been 
tfie channel by which hot springs, so common in countries 
of volcanoes and earthquakes, have made their way to the 
surface. For we know that the rents in which ores abound 
extend downward to vast depths, where the temperature of 
the interior of the earth is more elevated. We also know 
that mineral veins are most metalliferous near the contact of 
plutonic and stratified formations, especially where the former 
send veins into the latter, a circumstance which indicates an 
original proximity of veins at their inferior extremity to ig¬ 
neous and heated rocks. It is moreover acknowledged that 
even those mineral and thermal springs which, in the present 
state of the globe, are far from volcanoes, are nevertheless 
observed to burst out along great lines of upheaval and dis¬ 
location of rocks.* It is also ascertained that all the sub¬ 
stances with which hot springs are impregnated agree with 
those discharged in a gaseous form from volcanoes. Many 
of these bodies occur as vein-stones ; such as silex, carbonate 
of lime, sulphur, fluor-spar, sulphate of barytes, magnesia, 
oxide of iron, and others. 1 may add that, if veins have been 
filled with gaseous emanations from masses of melted matter, 
slowly cooling in the subterranean 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 reduc¬ 
tion in volume amounting to 10 per cent. The. slow crystal¬ 
lization, 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 reason¬ 
ing, 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 mineral veins would be identical. 
On the contrary, M. E. de Beaumont has judiciously 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 wa¬ 
ters 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, 
* See Dr. Daubeny’s Volcanoes. 
