A
TREATISE ON GEOLOGY.
CHAPTER VI.—continued
FLUVIATILE AND LACUSTRINE DEPOSITS.
WE now quit the marine deposits of tertiary and post tertiary
age, and fix our attention on a parallel series of
accumulations, in valleys, and ancient lakes, for the
most part under the influence of fresh waters. In
treating of formations in valleys, we cannot always confine
our illustrations to the operations of fresh waters,
because continued research appears, in several instances,
to show that what appeared at first to be due to lacustrine
fluctuation or river currents was really the effect
of water-movement in an ancient arm of the sea. This
result is quite to be expected. Valleys have been the
channels of strong sea currents before they were raised
above the ocean and filled with precipitations from the
air: valleys were sub aqueous before they became subaerial,
and in them we ought to find marks of marine
followed by other marks of fluviatile action.
When shells are absent (as they most frequently are), we may not always be able to distinguish between the beaches left by the retiring sea and the banks left by rapid inundations formerly flowing at higher levels.
