For half a century they have been the subject of patient
investigation and the boldest speculation. To them natural philosophers
have returned again and again to test their theories, and until they are
fully understood no steady or permanent advance can be made in the
various views which they have suggested to different observers. The
theory of the formation of lakes by barriers, presented by McCulloch and
Sir T. Lauder-Dick, that of continental upheavals and subsidences,
advocated by Sir Charles Lyell and Charles Darwin, that of inundations
by great floods, maintained by Professor H. D. Rogers and Sir George
Mackenzie, that of glacial action, brought forward by myself, have been
duly discussed with reference to this difficult case; all have found
their advocates, all have met with warm opposition, and the matter still
remains a mooted point; but the one of all these theories which shall
stand the test of time and repeated examination and be eventually
accepted will explain many a problem besides the one it was meant to
solve, and lead to farther progress in other directions.
I propose here to reconsider the facts of the case, and to present anew
my own explanation of them, now more than twenty years old, but which I
have never had an opportunity of publishing in detail under a popular
form, though it appeared in the scientific journals of the day.
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