s, that the earth's entire mass is at
least as rigid as steel, by supposing that a molten film rests between
the observed solid crust and the alleged solid nucleus. But be that
as it may, the theory that subterranean heat has been instrumental in
determining the condition of "primary" rocks, and in producing many
other phenomena of the earth's crust, has never been in dispute since
the long controversy between the Neptunists and the Plutonists led to
its establishment.

LYELL AND UNIFORMITARIANISM

If molten matter exists beneath the crust of the earth, it must contract
in cooling, and in so doing it must disturb the level of the portion of
the crust already solidified. So a plausible explanation of the upheaval
of continents and mountains was supplied by the Plutonian theory, as
Hutton had from the first alleged. But now an important difference
of opinion arose as to the exact rationale of such upheavals. Hutton
himself, and practically every one else who accepted his theory, had
supposed that there are long periods of relative repose, during which
the level of the crust is undisturbed, followed by short periods of
active stress, when continents are thrown up with volcanic suddenness,
as by the throes of a gigantic earthquake. But now came Charles Lyell
with his famous extension of the "uniformitarian" doctrine, claiming
that past changes of the earth's surface have been like present changes
in degree as well as in kind. The making of continents and mountains,
he said, is going on as rapidly to-day as at any time in the past. There
have been no gigantic cataclysmic upheavals at any time, but all
changes in level of the strata as a whole have been gradual, by slow
oscillation, or at most by repeated earthquake shocks such as are still
often experienced.

In support of this very startling contention Lyell gathered a mass
of evidence of the recent changes in level of continental areas. He
corroborated by personal inspection the claim which had been made by
Playfair in 1802, and by Von Buch in 1807, that the coast-line of Sweden
is rising at the rate of from a few inches to several feet in a
century. He cited Darwin's observations going to prove that Patagonia is
similarly rising, and Pingel's claim that Greenland is slowly sinking.
Proof as to sudden changes of level of several feet, over large areas,
due to earthquakes, was brought forward in abundance. Cumulative
evidence left it no longer open to question that suc