s was, we believe, laid down on ancient sea floors in the
remains of organic forms, which for their particular uses took the
materials from the old sea water. The vein-making action has served to
assemble these scattered bits of metal into the aggregation which
constitutes a workable deposit. In time, as the rocks wear down, the
materials of the veins are again taken into solution and returned to
the sea, thence perhaps to tread again the cycle of change.

In certain dikes, and sometimes also, perhaps, in lavas known as
basalts, which have flowed on the surface, the rock when cooling, from
the shrinkage which then occurs, has broken in a very regular way,
forming hexagonal columns which are more or less divided on their
length by joints. When worn away by the agencies of decay, especially
where the material forms steep cliffs, a highly artificial effect is
produced, which is often compared, where cut at right angles to the
columns, to pavements, or, where the division is parallel to the
columns, to the pipes of an organ.

What we know of dikes inclines us to the opinion that as a whole they
represent movements of softened rock where the motion-compelling agent
is not mainly the expansion of the contained water which gives rise to
volcanic ejection, but rather in large part due to the weight of
superincumbent strata setting in motion materials which were somewhat
softened, and which tended to creep, as do the clays in deep coal
mines. It is evident, however; it is, moreover, quite natural, that
dike work is somewhat mingled with that produced by the volcanic
forces; but while the line between the two actions is not sharp, the
discrimination is important, and occurs with a distinctness rather
unusual on the boundary line between two adjacent fields of phenomena.

* * * * *

We have now to consider the general effects of the earth's interior
heat so far as that body of temperature tends to drive materials from
the depths of the earth to the surface. This group of influences is
one of the most important which operates on our sphere; as we shall
shortly see, without such action the earth would in time become an
unfit theatre for the development of organic life. To perceive the
effect of these movements, we must first note that in the great
rock-constructing realm of the seas organic life is constantly
extracting from the water substances, such as lime, potash, soda, and
a host of