chap. x. Basin- like Plains of Chile. 285 



passes through a remarkable level gap in the moun- 

 tains, forming a true land-strait, and called the Angostura. 

 It then immediately expands into a second basin-formed 

 plain : this again to the south, contracts into another 

 land-strait, and expands into a third basin, which, how- 

 ever, falls suddenly in level about forty feet. This third 

 basin, to the south, likewise contracts into a strait, and 

 then again opens into the great plain of S. Fernando, 

 stretching so far south that the snowy peaks of the dis- 

 tant Cordillera are seen rising above its horizon as above 

 the sea. These plains, near the Cordillera, are generally 

 formed of a thick stratified mass of shingle ; * in other 

 parts, of a red sandy clay, often with an admixture of pu- 

 miceous matter. Although these basins are connected to- 

 gether like a necklace, in a north and south line, by smooth 

 land-straits, the streams which drain them do not all flow 

 north and south, but mostly westward, through breaches 

 worn in the bounding mountains ; and in the case of 

 the second basin, or that of Rancagua, there are two 

 distinct breaches. Each basin, moreover, is not drained 

 singly : thus, to give the most striking instance, but 

 not the only one, in proceeding southward over the 

 plain of Rancagua, we first find the water flowing 

 northward to and through the northern land-strait; 

 then, without crossing any marked ridge or water-shed, 

 we see it flowing south-westward towards the northern 

 one of the two breaches in the western mountainous 

 boundary ; and lastly, again without any ridge, it flows 

 towards the southern breach in these same mountains. 

 Hence the surface of this one basin-like plain, appear- 

 ing to the eye so level, has been modelled with great 

 nicety, so that the drainage, without any conspicuous 



1 The plain of S. Fernando has, according to MM. Meyen and 

 Gay (' Eeise,' &c. Th. i. ss. 295 and 298), near the Cordillera, an 

 upper step-formed plain of clay, on the surface of which they found 

 numerous blocks of rocks, from two or three feet long, either lying 

 single or piled in heaps, but all arranged in nearty straight lines. 



