CAUSES OF THE ARID COAST CLIMATE. 49 



culty presents itself. If the Andes act as a barrier 

 against the vapour-laden atmosphere of eastern tropical 

 America throughout Peru, Bolivia, and Northern Chili 

 why, it may be asked, do they fail to perform the 

 same function in Ecuador and Colombia ? Whence 

 the absolute contrast in point of climate that exists 

 between these regions ? Why is the littoral zone 

 between the Gulf of Guayaquil and that of Panama, 

 a distance of some eight hundred miles, not merely 

 less dry than that of Peru, but actually more moist 

 that most parts of the coast of Brazil or Guiana ? 



Some answer may, I think, be given to these ques- 

 tions. In the first place, comparing the orography 

 of Peru and Bolivia with that of Ecuador, some im- 

 portant differences must be noted. In Eastern Peru, 

 as is at once shown by the direction of the principal 

 rivers, we find no less than four parallel mountain 

 ranges, increasing in mean elevation as we travel from 

 east to west. The westernmost range, to which in Peru 

 the name Cordillera is exclusively applied, does not 

 everywhere include the highest peaks, but has the 

 highest mean elevation. The second range, exclu- 

 sively called Andes in Peru, rivals the first in height 

 and importance. I know of no collective names by 

 which to distinguish the third range, dividing the 

 valley of the Huallaga from that of the Ucayali, nor 

 the fourth range, forming the eastern boundary of the 

 latter stream. In South Peru and Bolivia the mountain 

 ranges are less regularly disposed, but cover a still 

 wider area ; and throughout the whole region it is 

 obvious that the warm and moist currents drifting 

 slowly westward have to traverse a zone of lofty 



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