1834] EXCAVATION OF THE VALLEY. 131 



\Vhat 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 vallej 

 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 ol 

 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 

 supposition would have been quite inadmissible; because, the same 

 step-like plains with existing sea-shells lyiag 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 

 corroded so vast an area and thickness of solid basaltic lava. Never- 

 theless, we must believe that the strata, undermined by the waters of 

 this ancient strait, were broken up into huge fragments, and these 

 lying scattered on the beach, were reduced first to smaller blocks, then 

 to pebbles, and lastly to the most impalpable mud, which the tides 

 drifted far into the Eastern or Western Ocean. 



With the change in the geological structure of the plains the character 

 of the landscape likewise altered. While rambling up some of the 

 narrow and rocky defiles, I could almost have fancied myself trans- 

 ported back again to the barren valleys of the island of St. Jago. 

 Among the basaltic cliffs I found some plants which I had seen 

 nowhere else, but others I recognized as being wanderers from Tierra 

 del Fuego. These porous rocks serve as a reservoir for the scanty rain- 

 water ; and consequently on the line where the igneous and sedimentary 

 formations unite, some small springs (most rare occurrences in Pata- 

 gonia) burst forth ; and they could be distinguished at a distance by 

 the circumscribed patches of bright green herbage. 



April 27th. The bed of the river became rather narrower, and henoa 

 the stream more rapid. It here ran at the rate of six knots an hour 



