VIII GEOLOGY OF PATAGONIA i8i 



whence the well-rounded pebbles of porphyry have been 

 derived : we may consider its average breadth as 200 miles, 

 and its average thickness as about 50 feet. If this great bed 

 of pebbles, without including the mud necessarily derived from 

 their attrition, was piled into a mound, it would form a great 

 mountain chain ! When we consider that all these pebbles, 

 countless as the grains of sand in the desert, have been derived 

 from the slow falling of masses of rock on the old coast-lines 

 and banks of rivers ; and that these fragments have been 

 dashed into smaller pieces, and that each of them has since 

 been slowly rolled, rounded, and far transported, the mind is 

 stupefied in thinking over the long, absolutely necessary, lapse 

 of years. Yet all this gravel has been transported, and prob- 

 ably rounded, subsequently to the deposition of the white beds, 

 and long subsequently to the underlying beds with the tertiary 

 shells. 



Everything in this southern continent has been effected on 

 a grand scale : the land, from the Rio Plata to Tierra del 

 Fuego, a distance of 1200 miles, has been raised in mass (and 

 in Patagonia to a height of between 300 and 400 feet), within 

 the period of the now existing sea -shells. The old and 

 weathered shells left on the surface of the upraised plain still 

 partially retain their colours. The uprising movement has 

 been interrupted by at least eight long periods of rest, during 

 which the sea ate deeply back into the land, forming at 

 successive levels the long lines of cliffs or escarpments, which 

 separate the different plains as they rise like steps one behind 

 the other. The elevatory movement, and the eating- back 

 power of the sea during the periods of rest, have been equable 

 over long lines of coast ; for I was astonished to find that the 

 step-like plains stand at nearly corresponding heights at far 

 distant points. The lowest plain is 90 feet high ; and the 

 highest, which I ascended near the coast, is 950 feet; and of 

 this only relics are left in the form of flat gravel-capped hills. 

 The upper plain of S. Cruz slopes up to a height of 3000 feet 

 at the foot of the Cordillera. I have said that within the 

 period of existing sea-shells Patagonia has been upraised 300 

 to 400 feet : I may add, that within the period when icebergs 

 transported boulders over the upper plain of Santa Cruz, the 

 elevation has been at least i 500 feet. Nor has Patagonia been 



