4 



University of California Publications. 



[Geology 



the factor of chance was the chief assurance that a representative 

 series of specimens would be assembled. The asphalt trap thus 

 so continuously baited throughout a considerable period of time 

 probably approaches very nearly in effect the efforts of a con- 

 scious and persistent collector of today. Toll was taken from 

 a large area if we may judge by the wide-ranging habit of 

 large vultures of today. Through the keen senses of these birds 

 the lure was effective at great distances. The trap was prac- 

 tically automatic, its demands insatiable, and its patience 

 unwearied. 



PEESEEVATION OF MATEEIAL. 

 The mode of preservation of the asphalt material was 

 especially favorable in the case of the condors. While still in 

 the live state the bone was plunged into the soft tar which 

 effectively sealed it from the air. With the maceration of soft 

 parts, the asphalt slowly penetrated the bone even to its minutest 

 structure. There was in most cases no period of weathering 

 before entombment, no erosion through stream or wind action, no 

 gnawing by rodents or crushing by wolf or fox. In asphalt 

 masses which were shallow at the time of entrapment the entire 

 body of the animal was not embedded at once. With birds, after 

 struggling ceased, the feather covering often may have kept the 

 carcass from complete submersion for some time. Cases are 

 noticeable among specimens recently entrapped in shallow out- 

 flows where the sacrum, dorsal region and the back of the skull 

 show extensive weathering while the limbs, sternum, and the basal 

 and facial parts of the skull were perfectly impregnated. That 

 such condition was less frequently the case with the condors 

 seems highly probable. Their natural prey, the large mammal, 

 was entrapped only in outflows of appreciable depth, a depth at 

 least sufficient to immerse the condor entirely. The violent 

 struggles of these powerful birds, of which more than one was 

 usually entrapped about a single carcass, would tend to effect the 

 speedy interment of the entire body. The slow sinking of the 

 mammalian carcass would tend to carry down the bird's body as 

 well. 



The large mammal, though entrapped in a comparatively 

 shallow tar pool, probably became submerged fairly rapidly on 



