SUMMARY. 287 



coast inland, westward toward the Andes, and also in crossing from north 

 to south any of the great transverse valleys. Such escarpments have a . 

 general trend somewhat parallel with that of the present coast line, but 

 extend inland for many miles along either side of the valleys of all the 

 more important water courses, as do also the present bluffs of the sea. 

 They are perhaps remnants of bluffs formed along the coast at different 

 stages during the former depression and late elevation of the land, which 

 would appear to have been intermittent and of which we have the last 

 stage exhibited in the present bluffs of the sea. Between the successive 

 escarpments, 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 vol- 

 canoes. Many of the latter rise high above the surrounding plain as 

 imposing landmarks, serving alike to guide the traveller and lend variety 

 to a rather monotonous landscape. These lava fields are most abundant 

 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 illimitable expanse, covered with 

 highly vesicular scoriae, while at other times the surface over large areas 

 is carved into a confusing labyrinth of deep, almost inaccessible canons. 

 In either case they present a most serious obstacle to the traveller. 



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 Straits of Magellan, 

 with several extinct volcanoes and resulting lava streams, which appear 

 to have been ejected at a comparatively recent date. In some few in- 

 stances the lavas of the great interior region extend westward quite to the 

 base of the Andes, but as a rule the surface of the plain, for a distance of 

 some thirty to forty miles eastward from the base of the mountains, is free 

 from lava. It has either never existed there, or has been entirely swept away, 

 or covered over by glacial detritus, as has been observed in some few 

 instances. 



The region between the western border of the lava beds and the foot- 

 hills of the Andes is by far the most fertile of the Patagonian plains. 

 Its surface, covered to a considerable depth with glacial deposits, presents 

 a series of ranges of low, rounded hills, left as terminal moraines by the 



