416 



Mr. Darwin on the Boulders 



sea ; its surface is somewhat but not greatly irregular, and the inequalities ap- 

 pear to have been caused chiefly by the denudation of loose matter from an irre- 

 gular field and hummocks of lava. The plain slopes very gently and with much 

 regularity to the Atlantic, where the sea- cliffs are about 800 feet high ; it rises 

 somewhat more abruptly towards the Cordillera, near which its height is above 

 3000 feet. The Cordillera in this latitude are not very lofty, the highest pinnacle 

 being only 6400 feet above the level of the sea. 



The accompanying section (Fig. 1.), exhibited on the banks of the river in the 



Fig. 1. 



Surface of plain, strewed with great angular erratic boulders, 1400 feet above the level of the sea. 



212 feet in thickness 



322 feet 



Shingle Bed. 



Basaltic Lava. 



^=^^^.^~.^-^j^'z~^_^ Thin, fine-grained, variously 



':^~-7^i~^^^ • " r— - ■ coloured strata. 



Bed of very small pebbles. 



■River Santa Cruz : here 280 feet above level of sea. 



longitude above referred to, will give a sufficient idea of the composition of the 

 plain on which the boulders lie. The upper bed is 212 feet in thickness, and ex- 

 hibits indications of being coarsely stratified. It is composed of well-rounded 

 shingle with great angular blocks strewed on the surface, and probably imbedded 

 (for owing to the state of the section I was unable to ascertain this point) in the 

 whole upper part of the stratum. The shingle bed is continued without interrup- 

 tion to the coast, and is there certainly of submarine origin. From its general 

 similarity throughout this space, I have no reason to doubt that the whole was 

 accumulated under similar circumstances. The lowest bed represented in the 

 section is composed of minute pebbles of the same varieties of rock, with the ex- 

 ception of those of basaltic origin, as the great boulders on the surface. The con- 

 trast in the means of transportal from the same source, afforded by the regularly 

 sifted minute pebbles of the lowest bed and the huge angular fragments of the up- 

 permost, separated by a great stream of lava and a deposit of fine sediment nearly 

 .500 feet thick, appears to be worthy of notice. 



The valley in which the Santa Cruz flows, widens as it approaches the Cordillera, 

 into a plain, in form like an estuary, with its mouth (see map, PI. XL.) directed to- 

 wards the mountains. This plain is only 440 feet above the level of the sea, and 



