64 BASIN OF THE LLA3TOS. 



the villages of Parapara and Ortiz. It grows very narrow 

 in several parts. Basins, the bottoms of which are perfectly 

 horizontal, communicate together by narrow passes with 

 steep declivities. They were, no doubt, formerly small lakes, 

 which, owing to the accumulation of the waters, or some 

 more violent catastrophe, have broken down the dykes by 

 which they were separated. This phenomenon is found 

 in both continents, wherever we examine the longitudinal 

 valleys forming the passages of the Andes, the Alps,* or 

 the Pyrenees. It is probable, that the irruption of the 

 waters towards the Llanos have given, by extraordinary 

 rents, the form of ruins to the Morros of San Juan and 

 of San Sebastian. The volcanic tract of Parapara and Ortis 

 is now only 30 or 40 toises above the Llanos. The eruptions 

 consequently took place at the lowest point of the granitic 

 chain. 



In the Mesa de Paja, in the ninth degree of latitude, 

 we entered the basin of the Llanos. The sun was almost at 

 its zenith ; the earth, wherever it appeared sterile and des- 

 titute of vegetation, was at the temperature of 48 or 50.t 

 Not a breath of air was felt at the height at which we were 

 on our mules; yet, in the midst of this apparent calm, 

 whirls of dust incessantly arose, driven on by those small 

 currents of air which glide only over the surface of the 

 ground, and are occasioned by the difference of temperature 

 between the naked sand and the spots covered with grass. 

 These sand- winds augment the suffocating heat of the air. 

 Every grain of quartz, hotter than the surrounding air, 

 radiates heat in every direction; and it is difficult to ob- 

 serve the temperature of the atmosphere, owing to these 

 particles of sand striking against the bulb of the thermo- 

 meter. All around us the plains seemed to ascend to the 

 sky, and the vast and profound solitude appeared like an 

 ocean covered with sea-weed. According to the unequal 

 mass of vapours diffused through the atmosphere, and the 

 variable decrement in the temperature of the different strata 

 of ail*, the horizon in some parts was clear and distinct ; in 

 other parts it appeared undulating, sinuous, and as if striped. 



* For example, the road from the valley of Ursern to the Hospice of 

 St. Gothard, and thence to Airolo. 



f A thermometer, placed in the sand, rose to 38*4 and 40" Reaumur. 



