THE ESTUARY 251 



which is held up by the tide, is seen also in the dis- 

 tribution of the greater depths ; they are no longer 

 uniformly found along the concave edge of the bends, 

 but are scattered irregularly. On the Parana Guazu 

 a depth of 130 feet has been ascertained. Its minimum 

 depth is twenty-two feet. 



The study of the estuary may be taken separately 

 from that of the river. It consists of three parts, 

 unequal in size, which open with increasing breadth 

 toward the Atlantic. The upper Rio de la Plata, 

 above Colonia and Punta Lara, has a width of about 

 thirty-five miles. The middle Plata, twice as wide, 

 extends to the latitude of Montevideo and Punta de 

 las Piedras. Then the outer harbour opens between 

 Maldonado and Punta Rasa. The water is still fresh 

 in the middle estuary up to eighty miles below Buenos 

 Aires. 



The bottom is alluvial except in the channels between 

 Martin Garcia and Colonia. 1 Differently from up the 

 river, where the channels have sandy bottoms, while 

 the banks are of fine clay, the channels of the estuary 

 have bottoms of mud and clay. In the outer harbour 

 the pilots recognize the approach of banks by the 

 sand which is brought up by the sounding-lead. The 

 action of the waves, which is not found in the river, 

 accumulates stuff of comparatively large size and 

 weight on the banks. 



In spite of the conclusions embodied in the nautical 

 instructions, which describe the estuary as a theatre 

 of rapid changes " occasioned by the continual deposits 

 of sand brought down by the Parana and the Uru- 

 guay/' * the estuary is, as a matter of fact, in a remark- 

 able state of equilibrium, and there is no trace of a 

 gradual accumulation of alluvia, or of important 



1 The granite which outcrops at Martin Garcia also forms the plat- 

 form of the English Bank in the outer harbour. 



3 The water in the estuary, worked up by waves and tide, contains 

 more sediment than the water of the river, 



