288 PROCEEDINGS OF THE AMERICAN ACADEMY 



the condition of the coast now prevailing at Conception Bay, and south 

 of it, — the coast range forming the archipelago, the Andes forming the 

 coast range, and the plains of the more northern regions becoming 

 changed to bays ; the immense basins succeeding each other towards 

 the north which form the so-called Desert of Atacama, the nitrate-beds, 

 the llanos of the coast, the pampas of Peru, through which the rivers 

 flowing to the west have cut deep valleys with more or less marked 

 terraces, showing the different periods of ascent in the elevation of the 

 continent. These plains are everywhere found, either between a 

 coast range and the base of the eastern talus of the Andes, or ex- 

 tending from the summit of the shore terrace, if we may so call it, 

 generally at a height of from 1,200 or 3,000 feet, sloj^ing to the 

 second terrace, with its base at an averasfe height of from 6,000— 

 7,000 feet, and then followed by a second and third more or less 

 indistinct terrace until we reach the main elevated plateau or basin 

 which lies between the eastern and western slope of the Andes. All 

 these basins show more or less distinctly the trace of their former 

 marine origin ; so that, if we are to judge from the presence of strictly 

 marine forms, the successive terraces developed on a magnificent scale 

 on the west coast of the Andes, with the interlying basins, we have 

 a fair presumption that the elevation of the Andes to their present 

 height has taken place at a comparatively recent date, and during their 

 upheaval the present nitrate district and saline deposits were left as 

 large lagoons during a considerable period, to judge from the great 

 thickness of the deposits found within their basins, all denoting the 

 presence of a comparatively quiet inland sea. 



Lake Titicaca itself must have, within a comparatively very recent 

 geological period, formed quite an inland sea. The terraces of its 

 former shores are everywhere most distinctly to be traced, showing 

 that its water-level must have had an elevation of 300 or -iOO feet at 

 least higher than its present level. Tliis alone would send its shores 

 far to the north in the direction of Pucara, forming a narrow arm 

 reaching up to S. Rosa. Lake Arapa is probably only an outlier of 

 the ancient lake, as well as several of the small lakes, now at a 

 considerable distance from the west shore. The immense plain of 

 Cabanillas, extending north beyond Lampa to Juliaca, only 100 or 

 120 feet above the lake at its highest point, was one sheet of water. 

 The terraces of the former shores are still very distinctly seen. The 

 eastern shores did not probably differ greatly from the present out- 

 line, though the peninsula of Achacache was probably an island. 

 The Bay of Puno must have been connected with the plains of Llave, 



