CLIMATE OF SOUTH AMEEICA. 27 



the action of the alternating north-east and south-east trade winds, both charged 

 with abundant moisture, by which the rocks have been ravined and their detritus 

 swept away. The lateral ridges formerly disj)osed parallel with the main range 

 have disappeared ; the geological strata, whose debris are still seen north and south 

 of the breach, have been destroyed and replaced by drift of more recent origin, 

 here and there heaped up around isolated knolls of the primitive formations. 

 Should the work of erosion continue, the time may be foreseen when the Cordil- 

 leras w^ill be completely pierced, when the Amazonian plains will be separated only 

 by a sill of low elevation from the Gulf of Guayaquil. 



But while the Andes have in this region been reduced to a narrow stem by 

 the destructive action of the rains, in Bolivia they have, on the contrary'-, been 

 maintained in their full amplitude, thanks to the shifting winds, which are here 

 deflected some to the north, some to the south, so that but little rain or snow falls 

 along their normal track. Farther south a fresh contrast corresponds with a fresh 

 change in the course of the aerial currents. Here the system is reduced to a 

 single range flanked at most with a few small parallel ridges ; it is intersected 

 by aee-p gorges and passes cut through the heart of the rocks, and is at last entirely 

 broken by the Strait of Magellan, Water was the agent by which the highlands 

 have thus been carved, hollowed out, and in places quite eaten away by the copious 

 rains accompanying the oceanic winds. 



At a former time, when the coast valleys were still filled with ice, glaciers also 

 contributed to modify the seaboard by preventing the deposit of alluvial matter, 

 and carrying seawards the detritus of all kinds. 



Climate. 



Taken as a whole, the South American continent enjoys a far more moderate 

 climate than the division of the globe on the ojDposite side of the Atlantic. Its 

 superiority in this respect must be attributed to the difPerence in the form of the 

 two continental masses. South America being much narrower, the moderating 

 influence of the surrovmding marine w^aters is more easily felt far inland. More- 

 over, the western continent is largely exposed to the action of the trade winds 

 which sweep up the broad valleys of the Orinoco and Amazons. In Africa, on the 

 contrary, the most elevated coast ranges are disposed along the shores of the 

 Indian Ocean, and thus intercept the winds blowing from the rainy quarter. The 

 northern section of this continent also lies to leew^ard of the huge mass of lands 

 formed by Europe and the whole of Asia, Thus it hajjpens that the north-east polar 

 wands passing over Turkestan, Persia, and Syria arrive almost comj)letely deprived 

 of moisture, and under their dry breath the summer heats become oppressive. 



In South America the line of greatest heat, which nearly coincides with the 

 seaboard between the Gulf of Uraba and Cape Sao Roque, scarcely represents an 

 average of more than 80° or 82° Fahr,, whereas in Africa the corresponding 

 isothermal traverses a zone where the normal temperature exceeds 86° Fahr,, and 

 where the heat is tempered by no sea breezes, as it is on the Colombian and 

 Venezuelan coastlands. 



