i68 The American Geologist. ^^^''■^^' ''-"'i- 
The northern of these two routes was the one traversed by 
Lieut. Musters more than thirty years ago; while the southern 
was chosen by Darwin, Moreno, and others. Few travelers 
diverged far from either of these natural highw'ays, so that 
much of the interior of Patagonia remained a terra incognita 
until quite recently. Within the last five years, however, our 
knowledge of the interior of this country has been very greatly 
increased. This has been accomplished chiefly by the explora- 
tions of the Argentine and Chilian boundary commissions, sup- 
plemented, perhaps, by the expeditions conducted by the pres- 
ent writer in southern Patagonia in behalf of Princeton Uni- 
versity. To these explorations must be accredited the discov- 
ery of many new^ lakes, rivers, mountains, and other geographic 
features, as will at once become apparent by a comparison of 
the sketch map accompanying this paper with any of the older 
maps of the same region. This is especially true of that region 
lying between the Rio Santa Cruz and the forty-sixth parallel 
of south latitude. 
It is not the purpose of the present paper to chronicle any 
of these discoveries, but rather to discuss the origin of some of 
the geographic features, and more especially of the lakes which, 
as will be seen by a glance at the accompanying map, consti- 
tute an important part of the physiography of this region. 
The lakes of southern Patagonia may be divided according 
to their origin into three classes, viz. : residual, glacial, and 
tectonic. Of by far the greater importance are the lakes of 
tectonic origin. By referring to the map, an intricate series 
of lakes will be seen to extend in a line approximating that of 
the seventy- second meridian of west longitude throughout the 
entire length of the region under discussion. The exceedingly 
irregular outline of nearly all these bodies of water distin- 
guishes them at once as true mountain lakes. Though the east- 
ern extremities of many of them occupy lateral valleys tha^ 
have been cut through the eastern range of the Andes and pro- 
ject well out into the great plain that extends from the moun- 
tains to the Atlantic, yet they one and all penetrate far to thj 
westward, extending quite through the eastern foot-hills and 
sending out numerous arms and ramifications into that lab\ - 
rinth of deep mountain gorges that separate the eastern lateral 
range of the southern Andes from the central and main range 
of the same mountain system. 
