290 PATAGONIAN EXPEDITIONS: GEOGRAPHY. 



Chico to within about sixty miles of the base of the Andes, and thence 

 bearing almost due north over the plains to the head of the Senguer 

 River and down the latter stream to the Chubut, never entering the 

 mountains at any point on their journey. 



Since our first visit, thanks to the energetic and capable staff of the 

 Argentine Boundary Commission, under the able direction of Dr. F. P. 

 Moreno, much of this region has been fully explored and admirably 

 charted, though there still remain vast tracts practically unknown and 

 offering a sure reward to the competent explorer. 



Since Dr. Moreno's publications are easily accessible, I shall not attempt 

 a detailed description of this interesting region, but shall briefly dis- 

 cuss the factors which have contributed to produce the existing unusual 

 drainage conditions. I am the more easily impelled to this course, since 

 some of the theories advanced by Dr. Moreno in explanation of certain 

 features, described by him, appear to me untenable. At any rate, they 

 are not supported by most of the observations made by myself in my 

 explorations. 



A study of the southern Andes, at any point, reveals the fact that they 

 are composed of three distinct, parallel ranges, separated by two deep, 

 narrow, longitudinal valleys. The middle of the three ranges is every- 

 where much higher than the two lateral ranges and may be reckoned as 

 the principal range of the Andes. The western lateral range is at present 

 partially submerged beneath the Pacific, but is still distinctly seen in the 

 chain of islands extending all along the western coast. The western of 

 the two longitudinal valleys is at 'present, throughout the greater extent 

 of Patagonia, entirely submerged beneath the sea, and is now represented 

 by the narrow system of rather deep channels that separates the islands 

 from the mainland and offers an almost continuously navigable inland 

 waterway, extending from the southernmost point of the Brunswick Pen- 

 insula to the forty-second parallel of south latitude, or throughout more 

 than twelve degrees, a distance of over 700 miles. 



The eastern lateral range of the Andes is seen in the foothills that rise 

 somewhat abruptly from the eastern plains to a height, in places, of some 

 6,000 or 7,000 feet. They are composed almost entirely of Secondary 

 and Tertiary sedimentary rocks, with occasional layers of intrusive basalts, 

 the whole thrown up in a somewhat complicated system of folds, usually 

 monoclines or anticlines, terminating toward the west in a lofty escarp- 



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