Ch. VII ] 



PARALLEL MOUNTAIN-CHAINS. 



131 









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a 





X 









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ft 



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some thousands of feet, was brought up again by a slow 

 movement in mass, during the era of the Eocene tertiary 

 formations, after which the whole sank down once more 

 several hundred feet, to be again uplifted to its present 

 level by a slow and often interrupted movement.* In a 



Pampean mud 



m w 



Megatherium My 



rupeds are buried. This mud contains in it recent species 

 of shells, some of them proper to brackish water, and is 

 believed by Mr. Darwin to be an estuary or delta deposit. 

 M. A. d'Orbigny, however, has advanced an hypothesis 



I 



M 



placement of the waters of the ocean, caused by the elevation 

 of the Andes, gave rise to a deluge, of which this Pampean 

 mud, which reaches sometimes the height of 12,000 feet, is 

 the result and monument. f 



In studying many chains of mountains, we find that the 

 strike or line of outcrop of continuous sets of strata, and 

 the general direction of the chain, may be far from rectilinear. 

 Curves forming angles of 20° or 30° may be found in the 

 same range as in the Alleghanies ; just as trains of active 

 volcanos and the zones throughout which modern earthquakes 

 occur are often linear, 



Nor are all of these, though contemporaneous or belonging 



without running in straight lines. 



means 



angles, the one to the other. 



Slow upheaval and subsidence. — Recent Observations have 

 disclosed to us the wonderful fact, that not only the west 

 coast of South America, but also other lar 



them several thousand miles 



some 



in circumference, such 



as 



are 



Scandinavia, and certain archipelagos in the Pacific, 

 slowly and insensibly rising; while other regions, such as 

 Greenland, and parts of the Pacific and Indian Oceans, in 

 which atolls or circular coral islands abound, are as gra- 

 dually sinking. That all the existing continents and sub- 



movements 



kind, continued throughout incalculable periods 



time 



* Darwin's Geology of South Ame- 

 rica, p. 248. London, 1846. 



f Systemes de Montagnes, p. 748. 



K 2 



