In the Structure of Scotland at the present day we
have a state of circumstances which appears to me capable
of throwing much light on the nature of these
eposits. We find there a series of narrow arms of the
sea stretching far into the land, and separated by rugged
and generally steep ranges of metamorphic or plutonic
rocks. They are all more or less silted up by sedimentary
matter, and near their mouths, especially where, as
is often the case, they are much contracted; we generally
find a bar, shallower than the remainder. At various
elevations above the sea-level again there is a series of
fresh-water lakes, differing little in aspect from the arms
• of the sea. We find also in many parts of the Highlands
of Scotland long valleys, nearly level, which are
filled with incoherent sedimentary deposits, and bounded
like the lochs by steep mountains. If these were for
merly arms of the sea, which by the elevation of the
land have been converted into dry land, then the freshwater
lakes probably occupy those parts of the narrow
channels which were originally deepest, or which, being
wider than the rest, have remained unoccupied by sedimentary
matter at the time of the elevation. In conformity
with this view we find that at the lower end of
these lakes the mountains generally approach very close
to one another.
If we were to suppose the gradual elevation of Scotland
to continue till the mountains attained an elevation
equal to that of the Himalaya, it is evident that a continued
series of marine sedimentary deposits would extend
from the summit to the sea-level, unless removed
by the action of streams or other ordinary causes. Some
of the valleys would be of considerable width, and would
contain marine fossils in great abundance; but in the
narrower mountain valleys the gravel and boulders would
be quite destitute of fossils. Here and there fresh-water
formations of partial extent would occur, but they would
be separated from one another by large tracts filled with
marine beds. The gradual elevation of the land would
bring to bear upon these incoherent strata the powerful
action of running water, which would remove portion
after portion, till at last deep valleys would be excavated,
and small patches only of the gravel and clay would remain
where the action of the streams was least powerful.
Such I conceive to be the present state of Tibet, but a
much more detailed investigation of that remarkable
country would be necessary, before this view can be regarded
in any other light than an hypothesis.
The causes by which the metamorphic rocks, which
must have been brought into their present remarkable
state at a great depth in the interior of the globe, acquired
their present configuration of mountain and valley,
form a question on which I am not now prepared
to enter. One continued process of elevation seems inadequate
to produce the observed effects; but however
numerous the alternations of elevation and depression
may have been, it is evident that the alluvial deposits at
present existing must all be referable to the last period
of elevation, as such incoherent strata could not withstand
the continued action of the sea.