o 



88 Summary on the Patagonian pake n. 



at the Rio Xegro. At Port S. Julian it is from 800 to 

 900 feet in thickness ; and at S. Cruz it extends, with 

 a slightly altered character, up to the Cordillera. From 

 its microscopic structure, and from its analogy with 

 other formations in volcanic districts, it must be con- 

 sidered as originally of volcanic origin : it may have 

 been formed by the long-continued attrition of vast 

 quantities of pumice, or, judging from the manner in 

 which the mass becomes, in ascending the valley of S. 

 Cruz, divided into variously coloured layers, from the 

 long-continued eruption of clouds of fine ashes. In 

 either case, we must conclude, that the southern volcanic 

 orifices of the Cordillera, now in a dormant state, were 

 at about this period over a wide space, and for a great 

 length of time, in action. We have evidence of this 

 fact, in the latitude of the Eio Xegro in the sandstone- 

 conglomerate with pumice, and demonstrative proof of 

 it. at S. Cruz, in the vast deluges of basaltic lava : at 

 this same tertiary period, also, there is distinct evidence 

 of volcanic action in Western Banda Oriental. 



The Patagonian tertiary formation extends continu- 

 ously, judging from fossils alone, from S. Cruz to 

 near the Rio Colorado, a distance of above 600 miles, 

 and reappears over a wide area in Entre Rios and Banda 

 Oriental, making a total distance of 1,100 miles; bat 

 this formation undoubtedly extends (though no fossils 

 were collected) far south of the S. Cruz, and, accord- 

 ing to M. d'Orbigny, 120 miles north of St. Fe. At 

 S. Cruz we have seen that it extends across the con- 

 tinent : beins" on the coast about 800 feet in thickness 

 (and rather more at S. Julian), and rising with the 

 contemporaneous lava-streams to a height of about 

 3,000 feet at the base of the Cordillera. It rests, 

 wherever any underlying formation can be seen, on 

 plutonic and metamorphic rocks. Including the newer 



