1S34.] EXCAVATION OF THE VALLEY. 181 



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 

 an arm 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 cut. 

 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- 

 roded so vast an area and thickness of solid basaltic lava. Ne- 

 vertheless, we must believe that the strata undermined by the 

 waters of this ancient strait, were broken up into huge frag- 



