Vol. 70.] THROUGH THE ANDES OF PEEU AXD BOLIVIA. 41 



formation of the Argentine Republic. These were kindly identified 

 for me by Dr. C. W. Andrews. They include the following : — 



Mandible of Mastodon (?) andium. a very old individual, with the surface 



of the teeth almost completely ground down. 

 Small mandible, with the two last milk-teeth (mm. 4), of Mastodon 



andium. 

 Right and left femurs, right radius, and a number of vertebrae of 



Mastodon ; small tusk of Mastodon (?). 

 Eight humerus and cervical vertebra of SceUdothevium sp. 

 Right radius of a small Megatheri ain-nke animal. 

 Claw of Megatherium (?). 

 Phalanges and metacarpal of Parahippidium. 



This high-level ten-ace has a wide distribution on each side of 

 the present river-valley, and may be regarded as having been in 

 .great j)art laid down under lacustrine conditions : for, at the time of 

 its formation, most of the country lying between the existing lakes 

 must have been continuously covered by a vast sheet of water. 

 Owing to the continued rise of the Eastern Cordillera, the rain- 

 fall became greatly diminished, evaporation set in, and the waters 

 gradually receded to their present limits, possibly aided in the 

 process by slight differential earth-movements. 



As the separation of the two present lakes was thus effected, 

 the Desaguadero, which now drains the intervening country, first 

 became confined to a definite channel. Its fall, however, must 

 have been so slight that constant flooding of the country was 

 inevitable, conditions which (even at the present day) are repeated 

 on a smaller scale during any pronounced wet season. 



The existence of lower terraces of more recent date may merely 

 indicate the recurrence of marked periods of excessive rainfall, or of 

 increased acceleration in the uplift of the country. Apart from 

 general considerations of topography, evidence for the former wide 

 extension of Lake Titicaca l during a comparatively-late period 

 of human existence has been obtained by Sefior A. Posnansky, of 

 La Paz, during his explorations among the ruins of Tiahuanacu. 



That ancient metropolis of a pre-Inca civilization, the megalithic 

 architecture of which is superior to anything found in the country at 

 the present day, is now situated more than 12 miles from the shores 

 of the lake, at an altitude of about 110 feet abo\ye its mean water- 

 level. That the waters of the lake extended to the city at the time 

 of its occivpation has been clearly proved by Senor Posnansky, 2 who 

 has discovered among its ruins elaborately-constructed moles and 

 artificial canals. This author has also good reasons for supposing 

 that the huge idols, and megalithic blocks used in the construction 

 of the buildings, were all transported to the locality by means of 

 rafts, or balsas, on the surface of the lake. 



1 Lake Titicaca now lies between latitudes 15° 20' and 16° 35' S. and 

 longitudes 70° 45' and 72° 10' W. It has been estimated to be about 140 

 miles long and 60 miles broad, and its greatest depth is said to be 1700 feet. 

 Its elevation above sea-level is about 12,616 feet. 



2 He estimates the age of Tiahuanacu as 11,000 years, from calculations 

 based on the astronomic orientation of the buildings. 



