314 PATAGONIA 



These extinct volcanoes are scattered over the plains in a not 

 entirely irregular manner. As before stated, they occur in groups, 

 there being iisuall}^ from three or four to as man}^ as a dozen in 

 each group within a radius of perhaps five miles. These crater 

 areas occur at irregular intervals on the plains of Patagonia from 

 near Cape Virgin at the eastern entrance to the Strait of Magellan 

 to as far north as the country visited by us, at least, and most 

 likel}^ for a considerably greater distance. For the most part, 

 they are found over an area extending parallel with the Cordil- 

 leras and distant from eighty to one hundred and twenty miles 

 from them. In places they rise but a few feet above the surface 

 of the surrounding country, and some of these may not be craters? 

 but simply elevations in the surface of the lava due to a heaping 

 up of the materials resulting from the intersection of two or more 

 streams while flowing. In many cases they I'ise several hundred 

 feet above the surrounding country, have immense craters or 

 bowls on their summits, and present unmistakable evidences of 

 having been active volcanoes within comparatively recent times. 



Whether these craters should be considered as lateral cones 

 dependent upon the greater volcanoes of the Cordilleras and as 

 having derived their molten materials from the same reservoir, 

 or as an independent s^^stem the materials of which were derived 

 from a distinct reservoir, it is difficult to say. From the sim- 

 ilarity of the basalts known to occur in the two regions, however, 

 I should be inclined to the former view. Between this series- of, 

 volcanoes and the Cordilleras, especially in the region lying south 

 of Santa Cruz river, there are wide, open plains, entirely unob- 

 structed by either extinct volcanoes or lava fields. 



Another interesting feature prominent in the topography of 

 Patagonia, especially in that part of the countr}^ b^ing east of the 

 crater region, is the occurrence of numerous shallow salt lagoons 

 at the bottoms of great depressions, or rather excavations, from 

 100 to 300 feet or more in depth, scattered over the surface of the 

 plains. I have described these lakes and discussed their origin 

 in a jn-evious paper already cited on the geology of the region. 



At a distance of ten or twenty miles from the Cordilleras the 

 shingle or bowlder formation increases greatly in thickness and 

 is composed of much coarser materials. Near the base of the 

 mountains the materials constituting this formation are not dis- 

 tributed in a uniform manner over the surface, so that the latter 

 loses its level, plain-like appearance, and presents numerous 

 small, rounded hillocks composed of heterogeneous masses of 



