THE VOYAGE OF THE BEAGLE 195 



What the thickness 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 

 moved? 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 Pata- 

 gonian 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 ter- 

 races the valley itself had been hollowed out. Although we 

 know that there are tides, which run within the Narrows 



