260 



DOMAIN III. ARGILLACEOUS. 



the mountains composed of primitive granite, 

 but as we proceeded, the granite became inter- 

 mixed with argillaceous slate of various colours ; 

 that however which chiefly predominates in the 

 Cordilleras is of a bluish cast, as far at least as 

 we had an opportunity of examining them. 

 Strata of lime-stone, and large masses of ferru- 

 ginous sand-stone, are in many places super- 

 incumbent on the argillaceous slate. We like- 

 wise found on the road coal, gypsum, and rock 

 salt; the last even on the summits of the most 

 elevated ridges." 



In the bed of the river Rosario he likewise 

 observed blue clay slate ; and he was surprised 

 to find the highest snow-capt mountains, within 

 nine miles of Potosi, covered with a pretty thick 

 Stratum of granitic stones, rounded by the action 

 of water; while there is a continual descent to 

 Tucuman, where the granitic ridge ends: and 

 from Tucuman to Potosi it consists of simple 

 clay slate. 



" The mountain Potosi, at whose foot the 

 city is built, resembles a sugar-loaf: it is almost 

 eighteen miles in circumference, and chiefly 

 composed of a yellow very firm argillaceous 

 slate, full of veins of ferruginous quartz, in 

 which silver ore, and sometimes brittle vitreous 

 ore, are found interspersed." 



On his journey from Potosi to Lima, he found 



