200 THE ANDES OF SOUTHERN PERU 



On the eastern margin of the volcanic country the flows thin 

 out and terminate on the summit of a limestone (Cretaceous) 

 plateau. On the western margin they descend steeply to the nar- 

 row west-coast desert. The greater part of the lava dips beneath 

 the desert deposits; there are a few intercalated flows in the 

 deposits themselves, and the youngest flows — limited in number — 

 have extended down over the inner edge of the desert. 



The immediate coast of southern Peru is not volcanic. It is 

 composed of a very hard and ancient granite-gneiss which forms 

 a narrow coastal range (Fig. 171). It has been subjected to very 

 long and continued erosion and now exhibits mature erosion forms 

 of great uniformity Of profile and declivity. 



From the outcrops of older rocks beneath the lavas it is pos- 

 sible to restore in a measure the pre-volcanic topography of the 

 Maritime Cordillera. In its present altitude it ranges from several 

 thousand to 15,000 feet above sea level. The unburied topography 

 has been smoothed out; the buried topography is rough (Figs. 29 

 and 166). The contact lines between lavas and buried surfaces in 

 the deep Majes and Cotahuasi valleys are in places excessively 

 serrate. From this, it seems safe to conclude that the period of 

 vulcanism was so prolonged that great changes in the unburied 

 relief were effected by the agents of erosion. Thus, while the 

 dominant process of volcanic upbuilding smoothed the former 

 rough topography of the Maritime Cordillera, erosion likewise 

 measurably smoothed the present high extra-volcanic relief in the 

 central and eastern sections. The effect has been to develop a 

 broad and sufficiently smooth aspect to the summit topography of 

 the entire Andes to give them a plateau character. Afterward the 

 whole mountain region was uplifted about a mile above its former 

 level so that at present it is also continuously lofty. 



The zone of most intense volcanic action does not coincide with 

 the highest part of the pre-volcanic topography. If the pre-vol- 

 canic relief were even in a very general way like that which would 

 be exhibited if the lavas were now removed, we should have to say 

 that the chief volcanic outbursts took place on the western flank 

 of an old and deeply dissected limestone range. 



