CHAPTER VI. 



THE ICHTHYIC FAUNA OF THE POTARO RIVER AND OF THE GUIANA PLATEAU. 



The Guiana Highland has a double interest. First, it is presumably one 

 of the oldest land-masses of South America, probably dating back to the time 

 when Africa and South America were connected by land or by a chain of islands. 

 Second, all of the rivers, so far as known, descend from the plateau over falls which 

 no fish can ascend, the most prominent of these being the Kaieteur, with a height 

 of seven hundred and forty-one feet, which divides the Potaro into an upper and 

 a lower course. 



Measurements made by Brown show that the Potaro, six hundred feet above 

 the fall, is four hundred and two feet wide and twenty and two-tenths feet deep. At 

 the brink of the fall it is three hundred and sixty-nine feet wide. These measurements 

 are taken at flood. The fall is seven hundred and forty-one feet high and in the 

 one thousand and twenty feet down stream from the kettle below the fall the 

 Potaro has a fall of eighty-one feet. The brink of the fall is about eleven hundred 

 and thirty feet above sea-level. 



The map of Anderson makes the Potaro about thirty miles long to Amatuk, 

 fifteen from Amatuk to the Kaieteur and fifteen from the Kaieteur to Holmia, meas- 

 ured in straight lines; it is twice as long or more following the course of the stream. 



The lower Potaro River, from the Kaieteur to near Potaro Landing, follows a 

 northwest to a northeast course, but from Potaro Landing to its mouth the course 

 is from west to east. 



At Tumatumari a dike of diabase five hundred yards wide crosses the Potaro 

 and causes the cataract at that point. Various dikes cross the Potaro between 

 Tumatumari and a point two miles above Potaro Landing, but none of these inter- 

 fere with navigation by launches. 



Above the mouth of the Curiebrong, 63 a northern tributary of the Potaro, there 

 are a series of dikes causing the Ichaura Rapids, and the Cobenatuk and Pakatuk 

 Cataracts. 



63 The Curiebrong River, a northern tributary of the Potaro, comes down from the table-land in the Amaila 



fall, one hundred and forty-four feet high, and then down an inclined plane for two miles to the level of the 



river below. 



94 



