30 STEPPES AND DESERTS. 



latter far surpass the Llanos of Venezuela in area; and their extent 

 is so great that, while their northern margin is bordered by palni 

 trees, their southern extremity is almost continually covered with ice. 

 The Tuyu, which resembles the Cassowary (the Struthio rhea), 

 is peculiar to these Pampas, which are also the haunt of troops of 

 dogs ( 15 ) descended from those introduced by the colonists, but 

 which have become completely wild, dwelling together in subterra- 

 nean hollows, and often attacking with blood-thirsty rage the human 

 race whom their progenitors served and defended. 



Like the greater portion of the desert of Sahara, ( 16 ) the north- 

 ernmost of the South American plains, the Llanos, are in the torrid 

 zone : during one-half of the year, they are desolate, like the Lybian 

 sandy waste; during the other, they appear as a grassy plain, re- 

 sembling many of the Steppes of Central Asia. ( ir ) 



It is a highly interesting though difficult task of general geography 



to compare the natural conditions of distant regions, and to represent 



by a few traits the results of this comparison. The causes which 



lessen both heat and dryness in the New World ( 18 ) are manifold, 



and in some respects as yet only partially understood. Amongst these 



i may be classed the narrowness and deep indentation of the American 



1 land in the northern part of the torrid zone, where consequently the 



\ atmosphere, resting on a liquid base, does not present so heated an 



ascending current; -the extension of the continent towards the 



jpoles; the expanse of ocean over which the trade-winds sweep 



[freely, acquiring thereby a cooler temperature; the flatness of the 



; eastern coasts; currents of cold sea-water from the antarctic regions, 



which, coming from the south-west to the north-east, first strike the 



coast of Chili in the parallel of 35 south latitude, and advance 



along the coast of Peru as far north as Cape Parina, and then turn 



f suddenly to the west; the numerous lofty mountain chains rich in 



springs, and whose snow-clad summits, rising high above all the 



strata of clouds, cause descending currents of cold air to roll down 



their declivities; the abundance of rivers of enormous breadth, 



which, after many windings, seek the most distant coast; Steppes 



which from not being sandy are less susceptible of acquiring a high 



degree of heat ; impenetrable forests occupying the alluvial plains 



situated immediately beneath the equator, protecting with their 



