42 ANNALS NEW YORE ACADEMY OF SCIENCES 



waterfalls, because they mark changes in geological structure which are 

 associated with profound changes in the environment of both aquatic and 

 terrestrial life. The waterfalls are also very important, because they have 

 been the source of some errors made by previous writers on the distribu- 

 tion of the aquatic forms. These errors have for the most part been due 

 to a lack of exact data concerning the waterfalls. 



The first of these great waterfalls is Paulo Affonso, which may be 

 designated as the "King of Cataracts." Paulo Affonso is found in the 

 lower course of Eio Sao Francisco and consists of a great series of rapids 

 above and below the falls. The volume of water which rushes over Paulo 

 its level 200 meters between Jatoba and Piranhas, a distance of 121 kilo- 

 meters by the railroad which connects the navigable portions of the river 

 above and below the real falls. In fact, the Eio Sao Francisco has lowered 

 Affonso is on the average about the same as that of Niagara.' Nearly all 

 this "muddy" water comes from the precipitation in the headwaters of 

 Eio Sao Francisco, its lower course from Joazeiro to Piranhas seldom 

 having rain. Consequently, during the dry season, Paulo Affonso is much 

 higher and more beautiful because the water is clearer. The relative 

 difference in the height of Paulo Affonso during the dry and flood seasons 

 is very great, because there are seven principal channels in the wide river 

 above the falls through which the water rushes into the long, narrow gorge 

 below the falls. I estimated the maximum height of a series of tumbles 

 at 143 feet, but the flood-marks in the gorge below the falls for the pre- 

 ceding year, which had only an average flood, would have reduced this 

 height at that time about 40 feet. 



Of the seven principal channels at Paulo Affonso, the southernmost one 

 on the Bahian side leaves the river about two miles above the falls and 

 gradually rushes into the gorge below the falls. The natives claim that 

 fishes swim up this channel during large floods, and I have no reason to 

 doubt their statement, because I found that the fishes above and below 

 the falls were almost identical, and because no great perpendicular fall 

 exists in the southernmost channel of Paulo Affonso. Naturally the ma- 

 rine forms which enter the mouth of this river are not found above the 

 falls, but Pacliyurus (Corvina) is; and since it is never found in either 

 Eio Novo or Eio Sapao, it must have originally passed Paulo Affonso, 

 when it left the sea and became a permanent fresh-water form. 



Finally, I may state that Professor Branner and one of his students, 

 Mr. E. Crandall, have recently reported marine Cretaceous above Paulo 

 Affonso at Jatoba. Consequently, Paulo Affonso did not exist at that 

 time, but most of the present fishes of the Sao Francisco also did not 

 exist at that epoch. There can be no doubt that Paulo Affonso is older 



