works which
engaged him till near the end of the year 1846. The first was his
"Geological Observations on South America", the second a recast of his
"Journal", published under the short title of "A Naturalist's Voyage
round the World".

The first of these works contains an immense amount of information
collected by the author under great difficulties and not unfrequently
at considerable risk to life and health. No sooner had Darwin landed
in South America than two sets of phenomena powerfully arrested his
attention. The first of these was the occurrence of great masses of red
mud containing bones and shells, which afforded striking evidence that
the whole continent had shared in a series of slow and gradual but
often interrupted movements. The second related to the great masses of
crystalline rocks which, underlying the muds, cover so great a part of
the continent. Darwin, almost as soon as he landed, was struck by
the circumstance that the direction, as shown by his compass, of
the prominent features of these great crystalline rock-masses--their
cleavage, master-joints, foliation and pegmatite veins--was the same as
the orientation described by Humboldt (whose works he had so carefully
studied) on the west of the same great continent.

The first five chapters of the book on South America were devoted to
formations of recent date and to the evidence collected on the east and
west coasts of the continent in regard to those grand earth-movements,
some of which could be shown to have been accompanied by
earthquake-shocks. The fossil bones, which had given him the first hint
concerning the mutability of species, had by this time been studied and
described by comparative anatomists, and Darwin was able to elaborate
much more fully the important conclusion that the existing fauna of
South America has a close analogy with that of the period immediately
preceding our own.

The remaining three chapters of the book dealt with the metamorphic and
plutonic rocks, and in them Darwin announced his important conclusions
concerning the relations of cleavage and foliation, and on the close
analogy of the latter structure with the banding found in rock-masses
of igneous origin. With respect to the first of these conclusions, he
received the powerful support of Daniel Sharpe, who in the years
1852 and 1854 published two papers on the structure of the Scottish
Highlands, supplying striking confirmation of the correctness of
Darwin's v