CHAP. Till. 



Elevation Gi r adtiaL 



215 



ceptible. The following table gives the average result 

 of many soundings off the Santa Cruz : — 



Under two miles from the shore, many of the pebbles were of 

 large size mingled with some small ones. 



Distance. 



Depth. 



Size of Pebbles. 



3 to 4 miles 



from the shore. 



11 to 12 fathoms. 



As large as walnuts ; mingled 











in every case with some 











smaller ones. 



6 to 7 miles 



do. 



17 to 19 



do. 



As large as hazel nuts. 



10 to 11 miles 



do. 



23 to 25 



do. 



From three to four tenths of 

 an inch in diameter. 



12 miles 



do. 



30 to 40 



do. 



Two-tenths of an inch. 



22 to 150 miles 



do. 



45 to 65 



do. 



One-tenth of an inch, to the 

 finest sand. 



I particularly attended to the size of the pebbles on 

 the 355 feet Santa Cruz plain, and I noticed that on 

 the summit-edge of the present sea-cliffs many were 

 as large as half of a man's head ; and in crossing from 

 these cliffs to the foot of the next highest escarpment, 

 a distance of six miles, I could not observe any increase 

 in their size. We shall presently see that the theory 

 of a slow and almost insensible rise of the land, will 

 explain all the facts connected with the gravel-capped 

 terraces, better than the theory of sudden elevations of 

 from one to two hundred feet. 



M. d'Orbigny has argued, from the upraised shells 

 at San Bias being embedded in the positions in which 

 they lived, and from the valves of the Azara labiata 

 high on the banks of the Parana being united and un- 

 rolled, that the elevation of Northern Patagonia and of 

 La Plata must have been sudden ; for he thinks, if it 

 had been gradual, these shells would all have been 

 rolled on successive beach-lines. But in protected bays, 

 such as in that of Bahia Blanca, wherever the sea is 

 accumulating extensive mud-banks, or where the winds 

 quietly heap up sand-dunes, beds of shells might assuredly 

 be preserved buried in the positions in which they had 

 lived, even whilst the land retained the same level ; any, 

 the smallest, amount of elevation would directly aid in 



