36 LAND MAMMALS IN THE WESTERN HEMISPHERE 



around the dwindling pools and fought for the last drops of 

 water," ^ Even in normal seasons springs and water holes 

 and the drinking places in streams are the lurking places 

 of beasts of prey and crocodiles, so that great accumula- 

 tions of bones are made around these spots. A succession 

 of unusually severe winters frequently leads to great 

 mortality among mammals, as happened in Patagonia in 

 the winter of 1899, when enormous numbers of Guanaco 

 perished of starvation on the shore of Lake Argentine, 

 where they came to drink. 



Bones which are exposed on the surface of the ground 

 decay and crumble to pieces in the course of a very few years ; 

 and if they are to be preserved as fossils, it is necessary that 

 they should be buried under sedimentary or volcanic deposits. 

 Several such modes of burial have been described in the fore- 

 going paragraphs, but there are other and equally important 

 methods, which remain to be considered. 



The deposits made by rivers are often extremely rich in 

 fossils, and most of the Tertiary formations of the Great Plains 

 are now ascribed to the agency of rivers. The flood-plain of 

 a stream, or that part of its basin which is periodically over- 

 flowed, is gradually built up by the layers of clay and silt 

 thrown down by the relatively still waters of the flooded 

 area, and scattered bones, skeletons or carcasses that may have 

 been lying on the ground before the freshet are buried in the 

 deposits. Bones covered up in this manner frequently show 

 the marks of teeth of rodents or carnivores which have gnawed 

 them when lying exposed. Deposits made in the stream- 

 channels, where the current was sw^iftest, are of coarser materials 

 such as gravel and sand, and these often contain the skeletons 

 of animals which were drowned and swept downward by the 

 flooded stream. When the Bison (the mistakenly so-called 

 Buffalo) still roamed in countless herds over the western plains, 

 immense numbers of them were drowned in the upper Missouri 

 1 J. W. Gregory, The Great Rift Vallej% p. 268. 



