232 PATAGONIAN EXPEDITIONS: GEOGRAPHY. 



range of the southern Andes. The western lateral range is, at present, 

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

 chain of islands that extend all along the western coast. The western of 

 the two longitudinal valleys is, at present, almost 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 affords 

 an almost continuously navigable inland waterway throughout the west 

 coast of this region. 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 six thousand or seven thousand feet. 



The lateral ranges are composed very largely of Secondary and Tertiary 

 sedimentary rocks, with occasional layers of basalt, usually intrusive. In 

 either instance, these are thrown up into a somewhat complicated system 

 of folds, which in the eastern range, at least, usually assume the form of 

 monoclines or anticlines. These terminate toward the west in a lofty 

 escarpment that overlooks the deep, narrow and irregular eastern longi- 

 tudinal valley between this eastern lateral range and the central main range 

 of the Andes. In this eastern longitudinal valley there is placed a chain 

 of most beautiful mountain lakes, extending from Lake Maravillo in south 

 latitude 51° 30' to the northernmost limits of Patagonia. To the south- 

 ward of Lake Maravillo the bottom of this eastern valley has not been 

 sufficiently elevated, and it is here occupied, not by fresh-water lakes, but 

 by an intricate system of inlets from the Pacific, as will be seen by a 

 glance at the accompanying map. 



The central and main range of the Andes differs from the two lateral, 

 ranges in being of greater altitude and of a more rugged nature. Unlike 

 the lateral ranges, it appears to be made up very largely of crystalline, 

 igneous and highly metamorphosed sedimentary rocks. Although more 

 continuous than either of the lateral ranges, it is pierced by numerous 

 deep chasms, through which either flow the rivers that drain the lakes 

 lying to the northward of Viedma and Argentino, or in the less elevated 

 region south of these lakes, extend the narrow channels that connect 

 Last Hope Inlet, Smith's Channels, Skyring and Otway Waters with 

 the Pacific. 



The Andes in this region, as elsewhere, are an extremely rugged and, 

 throughout long distances, an almost impenetrable mountain system. 

 For nearly a thousand miles to the northward of Lake Argentino, there 



