of high-level plateau fragments and lava fields which form the platform upon which the highest peaks stand. East of Iquique the sky line of the western summit of the Andes for at least forty miles is almost unbroken. The top, seen from the west, is as even as if cut by a knife drawn along the edge of a ruler. The elevation of the top averages about 12,000 to 14,000 feet. From this lofty platform the snow-capped Cordillera Sillilica rises several thousand fect, but it is only in this small elevation that the Andes are able to show a mountainous appearance. Their whole elevation above the sea has no expression in the relief of today. In the Puna de Atacama the average height of the basin floors is over 12,000 feet, and peaks and ridges rise to heights of only 1000 to 5000 feet about them. The Salar de Uyuni, at the southern end of the great basin of western Bolivia, is 12,000 feet above sea level, and there is little scope for the volcanoes on its border to make their distance above sea count in the relief.
The voleanic features of the Central Andes were preceded in their development by a land surface modeled to mature and even old forms over a vast extent of mountain country. There ensued wide and great uplift in the late Tertiary and Pleisto- cene periods. The elevation of the whole surface to higher levels was accompanied by the dissection of the mountain border as the draining streams had their gradients increased; and on the floors of the valleys the most striking features are the marks of recent and continuing dissection. Turbulent streams flowing over steep gradients dislodge and transport great quantities of waste, which is strewn over all the basin and valley floors. These marks of erosion at lower levels make more impressive the even crest lines of many plateau masses and the open and parklike character of the landscape. Grassy swards abound, and gentle, beautifully graded slopes. One’s imagination rather pictures the wilder mountain scenery of the lower level culminating in bold peaks, whereas quite the contrary is the case. ‘The top of the country has in many cases the gentlest relief. Where even crest lines are lacking there is at least a succession of graded mountain slopes showing late maturity of form. In other places all but fragments of older
