08 THE ANDES OF SOUTHERN PERU 



plateau that forms the greater part of the Maritime Cordillera, 

 that the snows massed on their shadier slopes have not yet ef- 

 fected any important topographic changes. The symmetrical 

 peaks of this class are in a few cases so very recent that they are 

 entirely nneroded. Lava flows and beds of tuff appear to have 

 originated but yesterday, and shallow lava-dammed lakes retain 

 their original shore relations. In a few places an older topog- 

 raphy, glacially modified, may still be seen showing through a 

 veneer of recent ash and cinder deposits, clear evidence that the 

 loftier parts of the lava plateau were glaciated before the last 

 volcanic eruption. 



The asymmetry of the peaks and ridges in the Maritime Cordil- 

 lera cannot be ascribed to the manner of eruption, since the con- 

 trast in declivity and form is persistently between northern and 

 southern slopes. Strong and persistent winds from a given direc- 

 tion undoubtedly influence the form of volcanoes to at least a 

 perceptible degree. In the case in hand the ejectamenta are 

 ashes, cinders, and the like, which are blown into the air and have 

 at least a small component of motion down the wind during both 

 their ascent and descent. The prevailing winds of the high 

 plateaus are, however, easterly and the strongest winds are from 

 the west and blow daily, generally in the late afternoon. Both 

 wind directions are at right angles to the line of asymmetry, and 

 we must, therefore, rule out the winds as a factor in effecting the 

 slope contrasts which these mountains display. 



It remains to be seen what influence a covering of vegetation 

 on the northern slopes might have in protecting them from ero- 

 sion. The northern slopes in this latitude (14° S.) receive a 

 much greater quantity of heat than the southern slopes. Above 

 18,000 feet (5,490 m.) snow occurs on the shady southern slopes, 

 but is at least a thousand feet higher on the northern slopes. It 

 is therefore absent from the northern side of all but the highest 

 peaks. Thus vegetation on the northern slopes is not limited by 

 snow. Bunch grass — the characteristic ichu of the mountain 

 shepherds — scattered spears of smaller grasses, large ground 

 mosses called yareta, and lichens extend to the snowline. This 



