PATAGONIA 313 



additional evidence in favor of m}^ view * tliat the crreut trans- 

 verse valleys of Patagonia were in existence prior to the last 

 submergence of this region in the Pliocene, and during which 

 submergence the marine Cape Fairweather beds were deposited. 

 During the elevation that caused the close of this submergence 

 there Avas distril)uted over this region the great Howlder or 

 Shingle formation (Tebuelche formation of An)eghino) of Pata- 

 gonia. These benches along the watercourses are not merely 

 river terraces formed of alluvial materials, but are composed of 

 the original strata constituting the Santa Cruz, Supra-Patagonian. 

 and Patagonian beds, as shown in numerous exposures. They 

 are often many miles in width, and 1 think show conclusively 

 that throughout certain periods during the elevation of this re- 

 gion these valleys formed deep embayments into which extended 

 the waters of the Atlantic. Some of the more imjjortant of these 

 valleys may even have formed straits connecting the Pacific and 

 Atlantic oceans, as has been claimed by Darwin. 



Another prominent feature over the Patagonian plains is the 

 occurrence of numerous volcanic cones, ai)pearing usually in 

 groups and at places remote from the Cordilleras. These craters, 

 although now extinct, have been active during comparatively re- 

 cent times, as is evidenced by the numerous small lava streams 

 to be found in many places, and which are seen to have tlowed 

 directly from some one of these craters down over the sides and 

 into the valleys of the present smaller watercourses, where they 

 have adapted themselves to the curves of the valleys and the 

 inequalities in the surface of the bottoms of the latter, and do 

 not extend into strata forming the sides of the valle3'S. Such 

 lava streams of comparatively recent origin always present an 

 irregular, hummocky surface, with numerous caverns, and are 

 composed almost always of very vesicular material. A splendid 

 example of such a lava stream may be seen in a small canon on 

 the southern side of the Rio Chico of the Gallegos river al)out 

 two miles below Palli Aike, near the point where the present na- 

 tional boundary line crosses the Chico. Probably these small 

 volcanoes were active throughout a considerable period in Ter- 

 tiary times, and largely furnished the materials of the Santa Cruz 

 beds. That they were active during the depositions of the Santa 

 Cruz beds is evidenced by the occurrence of lavas included be- 

 tween successive strata of those beds, which, owing to the ab- 

 sence of disturl)ance in the latter, can hardly be considered as 

 intrusive. 



♦ See "On tlie (ieology of Southern rntiigoniii," Am. .lour. Soi., Nov., 18'.t7, pp. .T.'T-3.M. 



