234 PATAGONIAN EXPEDITIONS: GEOGRAPHY. 



world, not even excepting the Tropics, is the annual amount of precipita- 

 tion greater, or the vegetation more abundant than along the entire extent 

 of the west coast of this region. 



Origin of the Longitudinal Valleys. I have already mentioned that 

 the three ranges, of which the Andes are in this region made up, are sep- 

 arated by two deep, narrow and irregular valleys. These valleys, unlike 

 the great transverse valleys of the plains region, are of tectonic, rather 

 than of erosive origin. They originated at the time of the birth of the 

 Andes and were caused by the unequal folding and flexing and by fault- 

 ing, which took place while the mountains were being elevated. Since 

 their origin the valleys have undoubtedly been much deepened and other- 

 wise altered by erosion, due to the action of both ice and water. 



Age of the Southern Andes. Notwithstanding that certain parts of 

 these mountains rise to an altitude of nearly or quite 15,000 feet, they are, 

 geologically speaking, of very recent origin and may be reckoned as among 

 the youngest, if not, in fact, the very youngest of the great mountain ranges 

 of the earth. In the bluffs to the south of Lake Pueyrredon I discov- 

 ered, at an altitude of 5,000 feet above the present level of the sea, 

 marine Tertiary deposits of late Pliocene age. Along their present west- 

 ern limits these deposits were inclined at a high angle, and with an 

 easterly dip, which indicated that formerly they had extended to a much 

 more considerable altitude. Moreover, the nature of the sediment of which 

 at this locality the Pliocene deposits are made up, as well as those con- 

 stituting the 2,500 feet of underlying marine and fresh-water beds of Oli- 

 gocene and Miocene age, not only points to a former much more west- 

 erly extension of those several formations, but necessarily precludes the 

 existence in the immediate vicinity, during the time of their deposition, 

 of any very great mountain ranges. For the rocks throughout the nearly 

 three thousand feet of Tertiary deposits are made up of uniformly fine- 

 grained sandstones and still finer clays, while beds of coarse conglomer- 

 ates, which are known everywhere to predominate in deposits laid down 

 in the vicinity of lofty mountains, are here conspicuous only by their 

 absence. Again, the great abundance of lignite and coal of late and 

 middle Tertiary age, taken in connection with the wide distribution of 

 the beds, which extend almost continuously throughout the Andean ranges, 

 save where they have been carried away by erosion, from Coronel and 

 Lota on the north to the south coast of Tierra del Fuego, and right in 



