Jan. 1834. 



GEOLOGY. 



203 



The whole series is horizontally stratified^ and I do not re- 

 collect ever seeing any signs of violence, not even such as a 

 fault. The gravel covers the entire surface of the land_, from the 

 Rio Colorado to the Strait of Magellan, a space of 800 miles, 

 and is one chief cause of the desert character of Patagonia. 

 Judging from a section across the continent at the St. Cruz 

 river, and from some other reasons, I believe the gravel beds 

 gradually thickening as they ascend, every where reach the 

 base of the Cordillera, It is to these mountains we must 

 look for the parent rocks, of at least a large portion of the 

 well-rounded fragments. I apprehend so great an area co- 

 vered by shingle, could scarcely be pointed out in any other 

 part of the world. 



Having said thus much of the constitution, let us look at 

 the external configuration of the mass. The level plains are 

 cut off along the whole line of coast by perpendicular cliffs, 

 which are necessarily of different altitudes, because any one of 

 the successive terraces, which, as I have already noticed, rise 

 like steps one above the other, may form the sea cUff. 

 These steps are often several miles broad; but from one 

 point of view I have seen four very distinct lines of escarp- 

 ment abutting one over the other. Having observed that 

 the plains appeared to run for great distances along the 

 coast at the same level, I measured barometrically the ele- 

 vation of some of them, and compared these measurements, 

 and took all those made by the officers employed in the 

 survey. I was astonished to find at how great distances, 

 even of 600 miles, plains occurred that had, within a few 

 feet of difference, the same elevation. I believe I can dis- 

 tinguish seven or eight distinct terraces which occur along 

 the line of coast, and which include heights between 1200 

 feet and the level of the sea. It will be understood that they 

 are not always present, for the lower ones have in some parts 

 been removed by the action of the sea sooner than in others. 

 When any broad valley enters the country, the terraces sweep 

 round and run up on each side ; in which case, the corre- 

 spondence on the opposite sides is beautifully illustrated. 



