IX 
EXCAVATION OF THE VALLEY 
191 
more than three or four miles down the river below their parent- 
source : considering the singular rapidity of the great body of 
water in the Santa Cruz, and that no still reaches occur in any 
part, this example is a most striking one, of the inefficiency of 
rivers in transporting even moderately-sized fragments. 
The basalt is only lava which has flowed beneath the sea ; 
but the eruptions must have been on the grandest scale. At 
the point where we first met this formation it was 120 feet in 
thickness ; following up the river-course, the surface impercep¬ 
tibly rose and the mass became thicker, so that at forty miles 
above the first station it was 320 feet thick. What the thick¬ 
ness may be close to the Cordillera, I have no means of knowing, 
but the platform there attains a height of about three thousand 
feet above the level of the sea : we must therefore look to the 
mountains of that great chain for its source ; and worthy of 
such a source are streams that have flowed over the gently 
inclined bed of the 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 evident 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 hundred feet, and a breadth varying from rather 
less than two miles to four miles ? The river, though it has so 
little power in transporting 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 an arm of the sea. It is needless in this 
work to detail the arguments 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 America 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 
