On the Geological Structure of the Amazons Valley. 359 



It is remarkable how very closely the measurements of the 

 body of this animal agree with those given by Dr. Hector, 

 in the ' Trans. N.Z. Institute,' vi. p. 85, of a porpoise from 

 Cook Straits that he refers to Delphinus Forsteri. The 

 Dusky-Sound specimen, however, differs considerably both 

 in colour and form from the figure of D. Forsteri in the 

 ' Voyage of the Erebus and Terror ;' while Dr. Hector says 

 that the Cook-Strait specimen " does not differ sufficiently 

 from that copied last year after Forster to make it worth 

 reproduction." 



The skeleton is being prepared for the Otago Museum. 



XLIX. — On the Geological Structure of the Amazons Valley. 

 By Professor James Orton*. 



The valley of the Amazons is a very shallow basin of vast 

 extent and of an oval shape, Avith the small end pointing east- 

 ward. Between December and June a large part of it resembles 

 a huge undrained swamp, and people sail half the year above 

 districts where for the other half they walk. Were the 

 forest removed from the Lower Amazons, a great mud flat 

 would be exposed (lower than the island of Marajd), threaded 

 by a network of deep channels, partially covered by every tide, 

 and deluged by the annual flood. From the marked feature 

 (first noticed by Chandless) that the tributaries enter the main 

 stream at a very acute ongle, and have exceedingly tortuous 

 courses, it is inferred that the rest of the valley is a nearly 

 level plain gently inclined from west to east, and with very 

 little slope on either side toward the centre of drainage. 



Between Borja and Para, a distance of 29°, the inclination 

 is only 500 feet. A section from Exaltacion, on the Upper 

 Madeira, which has the same altitude as Borja, to San Carlos, 

 on the Upper Negro (which is elevated only 212 feet above 

 the Atlantic), would show a depression at Fonte Boa, on the 

 Amazons, of only 150 feet in 1000 miles. The Negro is a 

 sluggish stream (San Carlos being on a level with Tabatinga); 

 the Napo is more rapid ; and the Pastissa is a torrent. In the 

 last thousand miles, the Madeira descends 430 feet, the Puriis 

 225, and the Ucayali 400 ; while the Huallaga has probably 

 a swifter current than any of the southern affluents. 



The basin of the Great Kiver is principally enclosed by the 



* Communicated by the Author, being Chapter XLI. of the forthcoming 

 new edition of his work on the Andes and the Amazons, 



