232 SANTA CRUZ, PATAGONIA. 



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 op- 

 posite sides of the valley, it was evident that the 

 strata once were united. What power, then, has 

 removed along a whole lino of country, a solid mass 

 of very hard rock, which had an average thickness 

 of nearly three hundred feet, and a breadth vary- 

 ing from rather less than two miles to four miles 1 

 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 foi'- 

 merly occupied by an arm of the sea. It is need- 

 less in this work to detail the arguments leading to 

 this conclusion, derived from the form and the na- 

 ture 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 1 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 in- 

 admissible, because the same step-like plains, with 

 existing sea-shells lying on their surface, which 

 front the long line of the Patagonian coast, sweep 



