METHODS — PAL^ONTOLOGICAL 31 



from the solution and re-deposition of the Hmestone of the 

 cavern-walls, by the agency of percolating waters. 



A mode of preservation which is unfortunately rare is ex- 

 emplified by the asphaltic deposits near Los Angeles, at Rancho 

 La Brea, which have been very fully described by Professor 

 J. C. Merriam of the University of California. The asphalt 

 has been formed by the oxidation and solidification of petro- 

 leum, which has risen up through the Pleistocene rocks 

 from the oil-bearing shales below. At one stage in the con- 

 version of petroleum into asphalt, tar-pools of extremely viscid 

 and adhesive character were, and still are, formed on the surface 

 of the ground ; and these pools were veritable traps for mam- 

 mals and birds and for the beasts and birds of prey which came 

 to devour the struggling victims. 



"The manner in which tar or asphalt pools may catch un- 

 suspecting animals of all kinds is abundantly illustrated at 

 the present time in many places in California, but nowhere 

 more strikingly than at Rancho La Brea itself, where animals 

 of many kinds have frequently been so firmly entrapped that 

 they died before being discovered, or if found alive were extri- 

 cated only with the greatest difficulty. As seen at this locality, 

 the tar issuing from springs or seepages is an exceedingly 

 sticky, tenacious substance which is removed only with the 

 greatest difficulty from the body of any animal with which it 

 may come in contact. Small mammals, birds, or insects 

 running into the soft tar are very quickly rendered helpless by 

 the gummy mass, which binds their feet, and in their struggles 

 soon reaches every part of the body. Around the borders of 

 the pools the tar slowly hardens by the evaporation of the 

 lighter constituents until it becomes as solid as an asphalt pave- 

 ment. Between the hard and soft portions of the mass there 

 is a very indefinite boundary, the location of which can often 

 be determined only by experiment, and large mammals in many 

 cases run into very tenacious material in this intermediate 

 zone, from which they are unable to extricate themselves." 



