1334.] EXCAVATION OF THE VALLEY. 18] 
sea to a distance of one hundred miles. At the first glance of 
the basaltic cliffs on the opposite sides of the valley, it was evi- 
dent that the strata once were united. What power, then, has 
removed along a whole line of country, a solid mass of very 
hard rock, which had an average thickness of nearly three hun- 
dred feet, and a breadth varying from rather less than two miles 
to four miles? The river, though it has so little power in trans- 
porting even inconsiderable fragments, yet in the lapse of ages 
might produce by its gradual erosion an effect, of which it is 
difficult to judge the amount. But in this case, independently 
of the insignificance of such an agency, good reasons can be 
assigned for believing that this valley was formerly occupied by 
anarm of the sea. It is needless in this work to detail the argu- 
ments leading to this conclusion, derived from the form and the 
nature of the step-formed terraces on both sides of the valley, 
from the manner in which the bottom of the valley near the 
Andes expands into a great estuary-like plain with sand-hillocks 
on it, and from the occurrence of a few sea-shells lying in the 
bed of the river. If I had space I could prove that South Ame- 
rica was formerly here cut off by a strait, joining the Atlantic 
and Pacific oceans, like that of Magellan. But it may yet be 
asked, how has the solid basalt been removed? Geologists 
formerly would have brought into play, the violent action of« 
some overwhelming debacle; but in this case such a supposition 
would have been quite inadmissible ; because, the same step-like 
plains with existing sea-shells lying on their surface, which front 
the long line of the Patagonian coast, sweep up on each side of 
the valley of Santa Cruz. No possible action of any flood 
could thus have modelled the land, either within the valley 
or along the open coast; and by the formation of such step- 
like plains or terraces the valley itself has been hollowed out. 
Although we know that there are tides, which run within the 
Narrows of the Strait of Magellan at the rate of eight knots an 
hour, yet we must confess that it makes the head almost giddy 
to reflect on the number of years, century after century, which the 
tides, unaided by a heavy surf, must have required to have cor- 
yoded so vast an area and thickness of solid basaltic Java. Ne- 
vertheless, we must believe that the strata undermined by the 
waters of this ancient strait. were broken up into huge frag- 
