220 Gravel Formation of Patagonia, paet n. 



but not invariably, containing grains of quartz. The 

 pebbles are embedded in a white gritty calcareous 

 matrix very like mortar, sometimes merely coating 

 with a whitewash the separate stones, and sometimes 

 forming the greater part of the mass. In one place I 

 saw in the gravel concretionary nodules (not rounded) 

 of crystallized gypsum, some as large as a man's head. 

 I traced this bed for forty-five miles inland, and was 

 assured that it extended far into the interior. As the 

 surface of the calcareo-argillaceous plain of Pampean 

 formation, on the northern side of the wide valley of 

 the Colorado, stands at about the same height with the 

 mortar-like cemented gravel capping the sandstone on 

 the southern side, it is probable, considering the appa- 

 rent equability of the subterranean movements along 

 this side of America, that this gravel of the Rio Negro 

 and the upper beds of the Pampean formation north- 

 ward of the Colorado, are of nearly contemporaneous 

 origin, and that the calcareous matter has been derived 

 from the same source. 



Southward of the Rio Negro, the cliffs along the 

 great bay of S. Antonio are capped with gravel : at San 

 Josef, I found that the pebbles closely resembled those 

 on the plain of the Rio Xegro, but that they were not 

 cemented by calcareous matter. Between San Josef 

 and Port Desire. I was assured by the Officers of the 

 Survey that the whole face of the country is coated with 

 gravel. At Port Desire and over a space of twenty-five 

 miles inland, on the three step-formed plains and in the 

 valleys, I everywhere passed over gravel which, where 

 thickest, was between thirty and forty feet. Here, as 

 in other parts of Patagonia, the gravel, or its sandy 

 covering, was, as we have seen, often strewed with re- 

 cent marine shells. The sandy covering sometimes 

 fills up furrows in the gravel, as does the gravel in the 



