stigations
which prove that the present thermal condition of the interior of the
earth implies either a heating of the earth within the last 20,000 years
of as much as 100 deg. F., or a greater heating all over the surface at
some time further back than 20,000 years, and then proceeds thus:--

"Now, are geologists prepared to admit that, at some time within the
last 20,000 years, there has been all over the earth so high a
temperature as that? I presume not; no geologist--no _modern_
geologist--would for a moment admit the hypothesis that the present
state of underground heat is due to a heating of the surface at so late
a period as 20,000 years ago. If that is not admitted, we are driven to
a greater heat at some time more than 20,000 years ago. A greater
heating all over the surface than 100 deg. Fahrenheit would kill nearly all
existing plants and animals, I may safely say. Are modern geologists
prepared to say that all life was killed off the earth 50,000, 100,000,
or 200,000 years ago? For the uniformity theory, the further back the
time of high surface-temperature is put the better; but the further back
the time of heating, the hotter it must have been. The best for those
who draw most largely on time is that which puts it furthest back; and
that is the theory that the heating was enough to melt the whole. But
even if it was enough to melt the whole, we must still admit some limit,
such as fifty million years, one hundred million years, or two or three
hundred million years ago. Beyond that we cannot go."[60]

It will be observed that the "limit" is once again of the vaguest,
ranging from 50,000,000 years to 300,000,000. And the reply is, once
more, that, for anything that can be proved to the contrary, one or two
hundred million years might serve the purpose, even of a thorough-going
Huttonian uniformitarian, very well.

But if, on the other hand, the 100,000,000 or 200,000,000 years appear
to be insufficient for geological purposes, we must closely criticise
the method by which the limit is reached. The argument is simple enough.
_Assuming_ the earth to be nothing but a cooling mass, the quantity of
heat lost per year, _supposing_ the rate of cooling to have been
uniform, multiplied by any given number of years, will be given the
minimum temperature that number of years ago.

But is the earth nothing but a cooling mass, "like a hot-water jar such
as is used in carriages," or "a globe of sandstone?" and has it