continuously for a great distance. The terrace B (north), at a point fifty-
five miles from the mouth of the river, was four miles in width; higher up
the valley this terrace (or at least the second highest one, for I could
not always trace it continuously) was about eight miles wide. This second
plain was generally wider than the lower ones--as indeed follows from the
valley from A (north) to A (south) being generally nearly double the width
of from B (north) to B (south). Low down the valley, the summit-plain A
(south) is continuous with the 840 feet plain on the coast, but it is soon
lost or unites with the escarpment of B (south). The corresponding plain A
(north), on the north side of the valley, appears to range continuously
from the Cordillera to the head of the present estuary of the Santa Cruz,
where it trends northward towards Port St. Julian. Near the Cordillera the
summit-plain on both sides of the valley is between 3,200 and 3,300 feet in
height; at 100 miles from the Atlantic, it is 1,416 feet, and on the coast
840 feet, all above the sea-beach; so that in a distance of 100 miles the
plain rises 576 feet, and much more rapidly near to the Cordillera. The
lower terraces B and C also appear to rise as they run up the valley; thus
D (north), measured at two points twenty-four miles apart, was found to
have risen 185 feet. From several reasons I suspect, that this gradual
inclination of the plains up the valley, has been chiefly caused by the
elevation of the continent in mass, having been the greater the nearer to
the Cordillera.

All the terraces are capped with well-rounded gravel, which rests either on
the denuded and sometimes furrowed surface of the soft tertiary deposits,
or on the basaltic lava. The difference in height between some of the lower
steps or terraces seems to be entirely owing to a difference in the
thickness of the capping gravel. Furrows and inequalities in the gravel,
where such occur, are filled up and smoothed over with sandy earth. The
pebbles, especially on the higher plains, are often whitewashed, and even