332 National Geographic Magazine. 



There are very few islands in this section of the river, the 

 banks are covered with huge trees matted with vines, and through- 

 out the lower half of the division, from Toro rapids to the mouth 

 of the San Carlos, the river is confined between steep hills and 

 mountains. 



As a result of the absence of considerable tributaries already- 

 noted, the fluctuations of this portion of the river conform closely 

 to those of the Lake, and consequently take place gradually and 

 are limited in range. 



Below the Rio San Carlos the San Juan changes its character 

 entirely. Its average width is twelve hundred and fifty feet, its 

 bottom is sandy, there are numerous islands, and the slope of the 

 river is almost uniformly one foot per mile. 



The discharge into this section of two large tributaries, the 

 San Carlos and the Sarapiqui, descending from the steep slopes of 

 the Costa Rican volcanoes, causes much more sudden and consid- 

 erable fluctuations of level than in the upper river. 



While the lower portion of the river and especially the delta 

 section presents vex-y interesting featui'es, yet the peculiar charm 

 of the river is in the upper section, and the exceptional advantages 

 it offers for obtaining miles of slack water navigation. This por- 

 tion of the river with the lake and the narrow isthmus between it 

 and the Pacific forms a trio of natural advantages for the con- 

 struction of a canal, the importance of which it would be difficult 

 to over estimate. 



About three miles below the mouth of the San Carlos, the Cario 

 Machado enters the San Juan on the north bank. This stream, 

 about one hundred feet wide and from eight to ten feet deep, is 

 the last of the mountain or torrential tributaries of the San Juan. 

 It can scarcely be said to have a valley, but occupies the bed of a 

 rugged ravine extending for several miles northerly and north- 

 westerly up into the easterly flank of the cordillera. Every 

 variety of igneous rock, from light porous pumice to dense metallic 

 green-black hypersthene andesite, may be picked up in the bed 

 of this stream. Agates also are common and there are occasional 

 masses of jasper. Farther up, frequent outcrops of trap in situ 

 occur, interspersed in some localities with numerous veins of 

 agate. 



Twelve miles below the Machado the San Francisco enters the 

 San Juan, This stream, with its several tributaries, drains a large 

 swampy valley sprinkled with irregular hummocks and hills. For 



