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 



