GEOLOGICAL IMPORTANCE OF SEDIMENTATION 331 



although not a professional geologist or geographer, made many acute 

 observations. His most significant statements in regard to the present 

 topic may be quoted as follows : 



The rivers which flow from the Cordillera proceed only from the melting of 

 the winter's snow, and bring down with them an amazing quantity of fine alluvial 

 mud. In their long passage through the mountains, and for some distance after 

 leaving them, the descent is so rapid that the great quantity of matter held in 

 suspension cannot subside. The Tunuyan, for instance, even as far as Coro 

 Corto, has as much mud in it as can be suspended in agitated water. This is 

 the case with the water supplying Mendoza, which none of the people can drink 

 without either filtering, or placing it for a long time in a state of quiescence; so 

 surcharged is it that they are obliged every day or two to clean out their irri- 

 gating channels, which would otherwise be filled with fine sand. If we take into 

 consideration the nature of the country to the southward, its long and almost 

 imperceptible descent towards the ocean, the immense bulk of alluvial matter 

 that must yearly be brought from the Cordillera, and which must somewhere 

 deposit itself, — we cannot but conclude that the rivers which may once have 

 flowed in deep and uninterrupted channels to the ocean, must, from such causes, 

 have had their beds raised in progress of time to the level of the surrounding 

 country: the continual shifting of their courses over level plains; the constant 

 accumulation of muddy detritus, must have effected the gradual disappearance of 

 navigable or continuous streams, and produced that series of swamps, and the 

 kind of country, which, according to the most credible accounts, exists throughout 

 the vast Pampa territory.' 



In the case of rivers which run through to the sea this process 

 of deposition comes to an end when they have built up this por- 

 tion of their courses to the necessary grade, unless a geosynclinal 

 warping takes place in front cf, and in line with, the neighboring 

 geanticline ; in which case the rivers may still continue* to build up an 

 extensive plain with a slope of from i to 10, or even 30, feet per mile. 

 This process will be favored when the area cf the plain is deficient in 

 rainfall, as is the case with the high plains of the United States and 

 Argentina; since under such climatic conditions there is no added 

 volume of river water to assist in carrying through to the sea the. 

 detritus obtained from the mountains. The same effect may take 

 place, however, where the climate of the plains is humid, provided 

 that the lessening of the grade is not fully compensated for by the 

 added volume of waters. 



I Travels in Chile and La Plata, by John Miers, in two volumes (London, 1826; 

 C. Baldwin, printer), Vol. I, p. 113. 



