284 Basin- like Plains of Chile. part n. 



this great range has been remarked by every author l 



since the time of Molina. Near the Cordillera it is 



composed of a stratified mass of pebbles of all sizes, 



occasionally including rounded boulders : near its 



western boundary, it consists of reddish sandy clay, 



containing some pebbles and numerous fragments of 



pumice, and sometimes passes into pure sand or into 



volcanic ashes. At Podaguel, on this western side of 



the plain, beds of sand are capped by a calcareous 



tuff, the uppermost layers being generally hard and 



substalagmitic, and the lower ones white and friable, 



both together precisely resembling the beds at Co- 



quimbo, which contain recent marine shells. Abrupt, 



but rounded, hummocks of rock rise out of this plain : 



those of Sta. Lucia and S. Cristoval are formed of 



greenstone-porphyry almost entirely denuded of its 



original covering of porphyritic clay stone breccia ; on 



their summits, many fragments of rock (some of them 



kinds not found in situ) are coated and united together 



by a white, friable, calcareous tuff, like that found at 



Podaguel. When this matter was deposited on the 



summit of S. Cristoval, the water must have stood 



946 feet 2 above the surface of the surrounding plain. 



To the south this basin-like plain contracts, and 



rising scarcely perceptibly with a smooth surface, 



1 This plain is partially separated into two basins by a range of 

 hills ; the southern half, according to Meyen (' Reise um Erde,' Th. i. 

 s. 274), falls in height, by an abrupt step, of between fifteen and twenty 

 feet. 



• 2 Or 2,690 feet above the sea, as measured barometrically by Mr. 

 Eck. This tuff appears to the eye nearly pure ; but when placed in 

 acid it leaves a considerable residue of sand and broken crystals, 

 apparently of feldspar. Dr. Meyen (' Eeise,' Th. i. s. 269) says, he 

 found a similar substance on the neighbouring hill of Dominico (and 

 I found it also on the Cerro Blanco), and he attributes it to the 

 weathering of the stone. In some places which I examined, its bulk 

 put this view of its origin quite out of question ; and I should much 

 doubt whether the decomposition of a porphyry would, in any case, 

 leave a crust chiefly composed of carbonate of lime. The white 

 crust, which is commonly seen on weathered feldspathic rocks, 

 does not appear to contain any free carbonate of lime. 



