Bowman — Geologic Relations of the Ouzeo Remains. 323 



been decayed before being caught by the aggrading stream, 

 their more fragile portions would be worn, though not without 

 respect to exposure of more projecting parts. The projecting 

 points are not necessarily the parts to decay more readily. It 

 may be safely argued from these two conditions also that the 

 bones were decidedly fresh at the time of burial, a condition 

 favoring long preservation. 



The bones lay in the zone of weathering, that is to say in 

 the zone between the surface and the ground water. At the 

 time the deposits were forming over them they undoubtedly 

 lay for a part of the time in the ground water and not in the 

 zone of weathering. When the deposits were later eroded and 

 the present ravines formed, the level of the ground-water zone 

 was lowered to the point where the bones once more lay in the 

 zone of weathering. The rate of weathering is, however, not 

 uniform in this zone ; it is accelerated by strong temperature 

 changes, by pronounced rise and fall of the ground water with 

 the wet and the dry seasons, and by a greater amount of capil- 

 lary water surrounding the soil grains. It is also hastened by 

 soil acids which in turn depend upon conditions of drainage 

 and of vegetable growth, being most abundant w^here the veg- 

 etation is abundant and the drainage poor. 



Organic material buried in the gravel deposits of the Cuzco 

 region at some depth in general would be well-preserved 

 because of (1) the thorough drainage, (2) the absence of impor- 

 tant amounts of vegetable acids in the soils owing to the rapid 

 drainage and the scanty vegetation, (3) the absence of strong 

 changes in temperature during the greater part of the period 

 of burial, and (4) occurrence during the later stages of erosion 

 on the face of a bluff where, though air could penetrate, the 

 bones were surrounded by material almost air-dry. 



The Nature of the Evidence. 



Yery few of the published arguments for the antiquity of 

 human remains rest, as in the present instance, upon physio- 

 graphic facts. It is, therefore, necessary to indicate the nature 

 of the physiographic evidence herein presented, its strong and 

 its weak points, and particularly, the necessity for its use. The 

 gravels have no fossils that in themselves throw light upon 

 the age of the beds ; the long erosion interval between the 

 inclined and dissected Tertiary strata and the glacial gravels 

 overlying them still further reduces the value of purely strati- 

 graphic evidence; and the gravels are in process of vigorous 

 dissection to-day, hence they were formed at some past time. 

 We have, then, to deal with a phase of recent geologic history, 

 without being able to rely upon any facts of structure or 



