44 (JEOGRAl'lIlc FEATURES OF SOLTUEIiX PATAGOXIA 



torns in most cases still flow the streams by which they have been 

 eroded ; though in some instances, like the Desire and Coy rivers, 

 there are now onl}-^ intermittent streams, while in the valley of San 

 Julian no stream at present ever flows, the waters of the original 

 stream having been captured long since at a distance of about 100 

 miles from the coast by a northern tributary of the Santa Cruz. The 

 latter, considering the volume of its waters, is much the most impor- 

 tant of all the rivers of the plains of southern Patagonia. 



Another feature characteristic of these plains is the series of escarp- 

 ments, often several hundred feet in height, that terminate a succes- 

 sion of terraces, encountered at varying elevations as one proceeds 

 from the coast inland westward toward the Andes, or also in crossing 

 from north to south any of the great transverse valle3'S. Such escarp- 

 ments have a general trend somewhat parallel with that of the ])res- 

 ent coast line, but extend inland for many miles along either side of 

 the valleys of all the more important watercourses, as do also the 

 present bluffs of the sea. They are perhaps remnants of bluffs 

 formed along the coast at different stages during the f(jrmer depres- 

 sion and late elevation of the land, which would appear to have 

 been intermittent and of which we have exhiljited in the present 

 bluffs of the sea the last stage. Between each successive escarpment 

 a narrow, level plain extends, gradually increasing in altitude to the 

 westward. 



In many places over the plains the sedimentary rocks are covered 

 with sheets of lava, which have usually had their origin in local dikes 

 or volcanoes. Many of the latter rise high above the surrounding 

 plain as imposing landmarks, serving alike to guide the traveler and 

 lend variety to a rather monotonous landscape. These lava fields 

 are most aljundant over the central interior region, midway between 

 the Andes and the coast, where they cover thousands of square miles. 

 In some instances they present a broad level surface of almost illim- 

 itaVjle exi)anse, covered with highly vesicular scorite. while at (jther 

 times the surface over large areas is carved into a confusing labyrinth 

 of deep, almost inaccessible, cafions. In either case they i)resent a 

 most serious obstacle to the traveler. 



While these lava beds are most frequent over the central interior 

 region, there is an important outlying area near the coast between 

 the mouth of the Gallegos River and the eastern entrance to the 

 Strait of Magellan, with several extinct volcanoes and resulting 

 lava streams, which appear to have been ejected at a comparatively 



