306 PATAGONIAN EXPEDITIONS: ZOOLOGY. 



northern regions becoming changed to bays ; the immense basins suc- 

 ceeding 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 

 extending from the summit of the shore terrace, if we may so call it, gen- 

 erally at a height of from 1200 to 3000 feet, sloping to the second ter- 

 race, with its base at an average height of from 6000-7000 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 dis- 

 tinctly the trace of their former marine origin, so that, if we are to judge 

 from the presence of strictly marine forms, the successive terraces devel- 

 oped 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 400 feet at least higher than 

 its present level. This 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 dis- 

 tinctly seen. The eastern shores did not probably differ greatly from the 

 present outline, though the peninsula of Achacache was probably an 

 island. The bay of Puno must have been connected with the plains of 

 Llave, and those back of Juli ; while from the lower lake, back of Aygache, 



