Darwin a shorter period than either of the other volumes of the series; his
diary records that the work was accomplished within ten months, namely,
between July 1844 and April 1845; but the book was not actually issued till
late in the year following, the preface bearing the date "September 1846."
Altogether, as Darwin informs us in his "Autobiography," the geological
books "consumed four and a half years' steady work," most of the remainder
of the ten years that elapsed between the return of the "Beagle," and the
completion of his geological books being, it is sad to relate, "lost
through illness!"

Concerning the "Geological Observations on South America," Darwin wrote to
his friend Lyell, as follows:--"My volume will be about 240 pages,
dreadfully dull, yet much condensed. I think whenever you have time to look
through it, you will think the collection of facts on the elevation of the
land and on the formation of terraces pretty good."

"Much condensed" is the verdict that everyone must endorse, on rising from
the perusal of this remarkable book; but by no means "dull." The three and
a half years from April 1832 to September 1835, were spent by Darwin in
South America, and were devoted to continuous scientific work; the problems
he dealt with were either purely geological or those which constitute the
borderland between the geological and biological sciences. It is impossible
to read the journal which he kept during this time without being impressed
by the conviction that it contains all the germs of thought which
afterwards developed into the "Origin of Species." But it is equally
evident that after his return to England, biological speculations gradually
began to exercise a more exclusive sway over Darwin's mind, and tended to
dispossess geology, which during the actual period of the voyage certainly
engrossed most of his time and attention. The wonderful series of
observations made during those three and a half years in South America