152 THE DESICCATION OF LATER GEOLOGICAL TIMES. 



Ancles of South America are much narrower, a section east and west across 

 them hardly averaging more than a third in length of that of a similar line 

 across the North American Cordilleras. Of course, under such conditions, it 

 is to be expected that the closed-basin region of South America, if one ex- 

 isted at all, would be much less extensive than our own. This is, in reality, 

 the fact, the drainage of a large part of the interior portions of the Andes 

 being very completely effected by the branches of the great rivers which 

 traverse the ranges by means of immense transverse breaks across them. 



Another important fact to be taken into consideration in this connection 

 is this : that the wide part of South America lies ■wholly in the tropical 

 regions, and is therefore subject to a very different regime from that of an 

 area within the temperate zone, like our own Cordilleras. The rain-fall of 

 the South American continent is very peculiarly affected, however, by the 

 position of the mountain ranges with reference to the trade-winds. Closed- 

 basin areas do exist in the Andes, and here, as we might have expected, 

 from analogy with the facts already described fiom our own western regions, 

 there are also clear evidences of desiccation in recent geological times. 



The closed basin occupied in part by Lake Titicaca is the most important 

 area of this kind in the Andes, and from the description furnished by 

 A. Agassiz* it is possible to state as an unquestionable fact, that the area of 

 water-surface not only of this lake, but of the region generally, has greatly 

 diminished in later times. The following quotation is offered in support of 

 this statement. 



" Lake Titicaca must have, within a comparatively very recent geological 

 period, formed quite an inland sea. The terraces of its Ibrmer 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 prob- 

 ably 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 to be seen The plains, now 



laid bare at the northern and western shores of Lake Titicaca, give us an 

 excellent idea the appearance the whole basin of the lake would present if 



* Hyilrogniphic Sketch of Lake Titii-aca ; Piuc. Am. AcaJ., Vol. XI. 1876, p. 2S8. 



