350 J. B. Hatcher — Geology of Southern Patagonia. 



showing stratification toward the bottom, where we found the 

 lignitized stems of small plants and parts of the skeletons of 

 two rodents. 



Important deposits of loess (?) were observed on the south 

 fork of the Rio Chico, which may be really aqueous deposits 

 belonging to the bowlder formation. They are best exposed 

 in the bluffs of the stream, where they show beautiful examples 

 of wind erosion, a section of which is shown here in fig. 8. 



Igneous Rocks. 



Darwin, on ascending the Santa Cruz River, was very much 

 struck by the beds of basalt which he observed capping the 

 bluffs on either side of the stream at places remote from the 

 Cordilleras, which he not unnaturally concluded had been the 

 source of all the basalts of this region. He considered these 

 basalts as examples of long distance lava flows over a bed only 

 very slightly inclined. Had Darwin gone overland into the 

 interior, instead of up the Santa Cruz River, he could not have 

 failed to discover that the source of these lava beds was not the 

 Cordilleras, but numerous small craters, found often in the 

 immediate vicinity. There is a group of these craters only a 

 few miles inland, near the mouth of the G-allegos River; they 

 are most numerous over an area about forty miles in breadth, 

 extending across the country from north to south, and distant 

 about 80 to 120 miles from the mountains. It is sometimes 

 possible to travel more than 100 miles between this chain of 

 craters and the Cordilleras without passing over a single lava 

 bed, especially in the district south of the Santa Cruz River. 



Notwithstanding that these craters exist in such great num- 

 bers all over the plains of Patagonia and penetrate right 

 through the strata of the Santa Cruz beds, yet there was 

 nowhere observable, in their vicinity, any faulting or disturb- 

 ance of the latter I can only account for this on the theory, 

 that these craters were in existence and active prior to and 

 during the deposition ol the Santa Cruz beds. I have no 

 doubt that they were the source from which was derived much 

 of the material of the latter deposits, since the latter are very 

 largely composed of volcanic conglomerates and ash, as stated 

 by Darwin. 



It is also evident that many of these craters continued active 

 long after the deposition of the Santa Cruz beds, for many of 

 the lava flows may still be seen, in places, descending from the 

 plains down over the slopes into the valleys of the water 

 courses, showing that the latter had been eroded prior to the 

 flow of streams exhibiting such conformation. 



Many of the craters show, especially in their lower parts, a 



