The transportal and origin of this vast bed of pebbles is an interesting
problem. From the manner in which they cap the step-formed plains, worn by
the sea within the period of existing shells, their deposition, at least on
the plains up to a height of 400 feet, must have been a recent geological
event. From the form of the continent, we may feel sure that they have come
from the westward, probably, in chief part from the Cordillera, but,
perhaps, partly from unknown rocky ridges in the central districts of
Patagonia. That the pebbles have not been transported by rivers, from the
interior towards the coast, we may conclude from the fewness and smallness
of the streams of Patagonia: moreover, in the case of the one great and
rapid river of Santa Cruz, we have good evidence that its transporting
power is very trifling. This river is from two to three hundred yards in
width, about seventeen feet deep in its middle, and runs with a singular
degree of uniformity five knots an hour, with no lakes and scarcely any
still reaches: nevertheless, to give one instance of its small transporting
power, upon careful examination, pebbles of compact basalt could not be
found in the bed of the river at a greater distance than ten miles below
the point where the stream rushes over the debris of the great basaltic
cliffs forming its shore: fragments of the CELLULAR varieties have been
washed down twice or thrice as far. That the pebbles in Central and
Northern Patagonia have not been transported by ice-agency, as seems to
have been the case to a considerable extent farther south, and likewise in
the northern hemisphere, we may conclude, from the absence of all angular
fragments in the gravel, and from the complete contrast in many other
respects between the shingle and neighbouring boulder formation.

Looking to the gravel on any one of the step-formed plains, I cannot doubt,
from the several reasons assigned in this chapter, that it has been spread
out and leveled by the long-continued action of the sea, probably during
the slow rise of the land. The smooth and perfectly rounded condition of