WESTERN ANDES: MARITIME CORDILLERA 201 



The volume of the lavas is enormous. They are a mile and a 

 half thick, nearly a hundred miles wide, and of indefinite extent 

 north and south. Their addition to the Andes, therefore, has 

 greatly broadened the zone of lofty mountains. Their passes are 

 from 2,000 to 3,000 feet higher than the passes of the eastern 

 Andes. They have a much smaller number of valleys sufficiently 

 deep to enjoy a mild climate. Their soil is far more porous and 

 dry. Their vegetation is more scanty. They more than double 

 the difficulties of transportation. And, finally, their all but un- 

 populated loftier expanses are a great vacant barrier between 

 farms in the warm valleys of eastern Peru and the ports on the 

 west coast. 



The upbuilding process was not, of course, continuous. There 

 were at times intervals of quiet, and some of them were long 

 enough to enable streams to become established. Buried valleys 

 may be observed in a number of places on the canyon walls, where 

 subsequently lava flows displaced the streams and initiated new 

 drainage systems. In these quiet intervals the weathering agents 

 attacked the rock surfaces and formed soil. There were at least 

 three or four such prolonged periods of weathering and erosion 

 wherein a land surface was exposed for many thousands of years, 

 stream systems organized, and a cultivable soil formed. No evi- 

 dence has been found, however, that man was there to cultivate 

 the soil. 



The older valleys cut in the quiet period are mere pygmies be- 

 side the giant canyons of today. The present is the time of domi- 

 nant erosion. The forces of vulcanism are at last relatively quiet. 

 Recent flows have occurred, but they are limited in extent and in 

 effects. They alter only the minor details of topography and 

 drainage. Were it not for the oases set in the now deep-cut can- 

 yon floors, the lava plateau of the Maritime Cordillera would 

 probably be the greatest single tract of unoccupied volcanic coun- 

 try in the world. 



The lava plateau has been dissected to a variable degree. Its 

 high eastern margin is almost in its original condition. Its west- 

 ern margin is only a hundred miles from the sea, so that the 



