60 CONTRIBUTIONS TO PLEISTOCENE GEOLOGY
It was these principles which formed the basis of one of the
great doctrines of Hutton's monumental Theory of the Earth.
Playfair in his memorable book, Illustrations of the Huttonian
Theory, laid down with great lucidity the doctrine that the
rivers had by their erosive power and wearing action cut out the
valleys in which they now flow, and that the terraces or haughs
occurring on the sides of those valleys "are always proofs of the
waste and detritus produced by the river, and of the different
levels at which it has run." Since Playfair's time the more modern
upholders of Hutton's Theory have thrown great light upon
the various processes of river action, for whereas Hutton and
Playfair recognized the two grand principles of erosion and
deposition—of destruction and construction—they left practically
the whole of the details of the methods by which rivers
accomplish the work implied by those principles to be
described by other writers.
There can be no doubt that such erosion and deposition
connected with the winding and shifting of rivers suffices to
terrace the sides of their valleys. The terraces thus produced
are of several different types, and an admirable description of
them and their production was given by the late Mr. Hugh
Miller, jun., in his paper on the subject.1 It occasionally
happens, as Mr. Barrington Browne has described in the case of
the Amazon,2 that a river has during the progress of a vast
period of time shifted its course relatively at different points from
one side of its valley to the other, and thence back again. During
the interval which elapsed between the commencement and the
end of this protracted itinerary the river has been able to excavate
its channel to a far lower level than it had at first attained, and
consequently high up above the present channel, that is, on the
summit of the bank now being eaten into by the stream, will be
found the sediments which were deposited when the river had
once before flowed on the same side of the valley as it does
to-day.
Further, if we journeyed as the stream has done across the
valley, it is obvious that a series of deposits, intermediate in
altitude and in age between those ancient beds on the top of the
cliff and those which are now being laid down at its foot, would
1 Miller, Proc. Roy. Phys. Soc, Edin.. 1883, pp. 563—306.
2 Browne, Q. J. Geol. Soc, xxx., p. 333.