GEOLOGY OF THE LOWER AMAZON REGION 727 



gression there existed on the north side of the Guiana-Amazonian 

 land an old ocean strait extending over the Antilles, Venezuela, and 

 Columbia, to Peru. These are the Hippurite and Actaonella deposits 

 of Jamaica, the Gault and Lower Senonian of Venezuela, the Chalk 

 of Columbia, and the Upper Albian and Senonian of Peru. 



During the Tertiary the lower Amazon region remained land — i. e., 

 in the sense that it was not covered by the sea. The land waters con- 

 tinued to flow into the Pacific until Miocenic time, before the elevation 

 of the Cordilleras. In Middle Miocenic, the drainage was reversed, 

 lakes were formed, and finally a great lake covered the entire low 

 land between the Guiana-east-Amazon land on the east and the con- 

 tinually rising Cordilleras in the west, extended from Naute to Madeira 

 and from middle Parii to the Rio Negro. 



As a result of the continued elevation of the Cordilleras, the con- 

 nection between the Atlantic and the Pacific ceased in young Mio- 

 cenic time, and South America was then united with North America. 

 At this time Mastodon came from the north, as M. humholdti and M. 

 andium are found in the young Miocenic deposits of the middle 

 Amazon (Paru) region. 



"A retrospect over the present short presentation brings out most 

 clearly that the entire younger geological history of the lower Amazon 

 region, beginning with about the Permian, took place upon the land. 

 From this it follows that the local floras and faunas must have been 

 continuous and that they were spared great disturbances. This, in 

 fact, is shown by certain details, as the very ancient forms of fishes, 

 as Lepidosiren, have been enabled to continue to the present. This 

 hypothesis,, however, will become clearer when the organic remains 

 of the Tertiary deposits are better known. Their gathering and study 

 are now the most important task of geological investigation on the 

 sunny shores of the majestic great river: Amazon." 



PART II. SILURIC SYSTEM 



The Siluric strata, Derby (6:167) states, 



appear on the Guiana side in a belt of a few miles in width, which extends in the 

 direction east -west for a considerable distance, if not along the whole southern 

 margin of the metamorphic region to Guiana. They have been recognized on 

 the Trombetas, Curua, and Maecurti, and .... I judge that they extend east- 

 ward nearly to the Atlantic. I estimate the total thickness of the series at about 

 1,000 feet. 



