onsistent with M. E. de Beaumont's hypothesis, which requires a shell
not more than thirty miles thick, or even less. Mr. Hopkins admits that
the exterior of the planet, though solid as a whole, may contain within
it vast lakes or seas of lava. If so, the gradual fusion of rocks, and
the expansive power of heat exerted for ages, as well as the subsequent
contraction of the same during slow refrigeration, may perhaps account
for the origin of mountain-chains, for these, as Dolomieu has remarked,
are "far less important, proportionally speaking, than the inequalities
on the surface of an egg-shell, which to the eye appears smooth." A
"centripetal force" affecting the whole planet as it cools, seems a
mightier cause than is required to produce wrinkles of such
insignificant size.

In pursuing his investigations, M. E. de Beaumont has of late greatly
multiplied the number of successive periods of instantaneous upheaval,
admitting at the same time that occasionally new lines of upthrow have
taken the direction of older ones.[250] These admissions render his
views much more in harmony with the principles advocated in this work,
but they impair the practical utility of parallelism considered as a
chronological test; for no rule is laid down for limiting the interval,
whether in time or space, which may separate two parallel lines of
upheaval of different dates.[251]

Among the various propositions above laid down (p. 164), it will be
seen that the sudden rise of the Andes is spoken of as a modern event,
but Mr. Darwin has brought together ample data in proof of the local
persistency of volcanic action throughout a long succession of
geological periods, beginning with times antecedent to the deposition of
the oolitic and cretaceous formations of Chili, and continuing to the
historical epoch. It appears that some of the parallel ridges which
compose the Cordilleras, instead of being contemporaneous, were
successively and slowly upheaved at widely different epochs. The whole
range, after twice subsiding some thousands of feet, was brought up
again by a slow movement in mass, during the era of the Eocene tertiary
formations, after which the whole sank down once more several hundred
feet, to be again uplifted to its present level by a slow and often
interrupted movement.[252] In a portion of this latter period the
"Pampean mud" was formed, in which the Megatherium mylodon and other
extinct quadrupeds are buried. This mud contains