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observations made on the Rio Negro, and on the line of a transverse 

 section from Rio Santa Cruz to the base of the Cordilleras. These 

 exhibit the structure of what Mr. Darwin calls the great southern ter- 

 tiary formations of Patagonia. 



The lowest of these formations appears to be of great extent and 

 thickness, and in one instance was found to contain a bed of an- 

 cient lava, which seemed to mark the commencement of the eruption 

 from the craters of the great chain of the Andes. It is character- 

 ized by a great oyster, and by other shells and corals, some of which 

 belong to species now living on the neighbouring coasts. Over it is a 

 deposit which xMr. Darwin describes as chiefly composed of rolled por- 

 phyry pebbles, which he had himself traced for more than 700 miles. 

 Overlying all the rest, and at a greater elevation above the level of 

 the sea, were beds of recent shells, identical in species with the lit- 

 toral shells of the neighbouring shores. Among these, he more espe- 

 cially notices a widely extended bed of Muscles, which still retain 

 their blue colour, and emit an animal smell when thrown in the fire. 

 From these facts, he thinks the tertiary deposits of Patagonia may be 

 separated into distinct periods, somewhat similar to those derived by 

 Mr. Lyell from a comparison of the newer deposits of Europe : and 

 in making the transverse section, bethought that he saw traces in the 

 valley of Santa Cruz of an ancient channel, which must have traversed 

 a great portion of the south part of the continent before the elevation 

 of the tertiary groups. 



In noticing the groups on the western side of the Andes, he de- 

 scribes an old tertiary deposit (eocine or miocene ?) south of Rio 

 Maypo, and abundance of recent shells 1300 feet above the same level. 

 He also describes the association of lava with beds containing recent 

 shells in the island of Chiloe. Among other facts, he notices the ap- 

 pearance of pitchstone among the beds of lava, and the occurrence of 

 a forest growing over a bed of recent oysters 350 feet above the ac- 

 tual level of the sea. All these recent shells are the littoral shells of 

 the neighbouring shores ; from which he concludes that the elevation 

 must have been gradual, or by successive hitches, similar to those by 

 which the coast of Chili and, more recently, the coast of Chiloe have 

 been unquestionably elevated. 



In addition to these very remarkable notices, Mr. Darwin mentions 

 other tertiary deposits at Chiloe and Conception, composed of beds 

 of sandstone and carbonaceous shale without shells, but containing 

 many silicified trunks of dicotyledonous trees, and alternating with 

 beds of lava. 



During the progress of the four years' survey (in addition to the 

 traverse above mentioned), Mr. Darwin crossed from Rio Negro to 

 Buenos Ayres by Sierra de la Ventana, a chain almost unknown to 

 geographers. He found two immense collections of large bones (of 

 Mastodons) near Santa Fe, but in a condition not to admit of their 

 being removed. He also found bones of a species of Mastodon at 

 Fort St. Julian, S. lat. 50°, and more than 600 miles from the former 

 localities. In one instance the bones appear to have been associated 

 with marine shells. In the gravel of Patagonia he also found many 



