hich lifted up the edges shows itself
in volcanoes, and as the edges have been raised, the middle part of
the mass has gone down. In other words, the facts of physical geography
precisely and exactly correspond with the hypothesis which accounts for
the infinite varieties of coral reefs.

One other point, before I conclude, about this matter. These reefs, as
you have just perceived, are in a most singular and unexpected manner
indications of physical changes of elevations and depressions going on
upon the surface of the globe. I dare say it may have surprised you to
hear me talk in this familiar sort of way of land going up and down;
but it is one of the universal lessons of geology that the land is
going down and going up, and has been going up and down, in all sorts
of places and to all sorts of distances, through all recorded time.
Geologists would be quite right in maintaining the seeming paradox that
the stable thing in the world is the fluid sea and the shifting thing is
the solid land. That may sound a very hard saying at first, but the more
you look into geology, the more you will see ground for believing that
it is not a mere paradox.

In an unexpected manner, again, these reefs afford us not only an
indication of change of place, but they afford an indication of lapse of
time. The reef is a timekeeper of a very curious character; and you can
easily understand why. The coral polype, like everything else, takes a
certain time to grow to its full size; it does not do it in a minute;
just as a child takes a certain time to grow into a man so does the
embryo polype take time to grow into a perfect polype and form
its skeleton. Consequently every particle of coral limestone is an
expression of time. It must have taken a certain time to separate the
lime from the sea water. It is not possible to arrive at an accurate
computation of the time it must have taken to form these coral islands,
because we lack the necessary data; but we can form a rough calculation,
which leads to very curious and striking results. The computations of
the rate at which corals grow are so exceedingly variable, that we must
allow the widest possible margin for error; and it is better in this
case to make the allowance upon the side of excess. I think that anybody
who knows anything about the matter will tell you that I am making a
computation far in excess of what is probable, if I say that an inch of
coral limestone may be added to one of t