614 
ELEMENTS OF GEOLOGY. 
the more will they cool, till they acquire the average tempera¬ 
ture of springs, being in that case chiefly charged with the 
most soluble substances, such as the alkalies, 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 dis¬ 
tribution of metallic matter in veins may be referred to ordi¬ 
nary chemical action, or to those variations in temperature 
which waters holding the ores in solution must undergo, as 
they rise upward 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 con¬ 
taining ores of lead, zinc, and copper, but chiefly lead, trav¬ 
erse 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 green¬ 
stone, or ‘‘ toad-stone,” as it is called provincially. 'Not that 
the original Assure is narrower where the greenstone occurs, 
but because more of the space is there filled with vein-stones, 
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 re¬ 
spect 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. 
Supposed relative Age of the different Metals. —After duly 
reflecting on the facts above described, we can not doubt 
that mineral veins, like eruptions of granite or trap, are re¬ 
ferable to many distinct periods of the earth’s history, al¬ 
though it may be more difficult to determine the precise age 
of veins; because they have often remained open for ages, 
and because, as we have seen, the same fissure, after having 
been once filled, has frequently been re-opened or enlarged. 
But besides this diversity of age, it has been supposed by 
some geologists that certain metals have been produced exclu¬ 
sively in earlier, others in more modern times; that tin, for 
example, is of higher antiquity than copper, copper than lead 
or silver, and all of them more ancient than gold. I shall 
first point out that the facts once relied upon in support of 
some of these views are contradicted by later experience, 
and then consider how far any chronological order of ar¬ 
rangement can be recognized in the position of the precious 
and other metals in the earth’s crust. 
* Bulletin, iv., p. 1278. 
