76 University of California Publications in Geology [VOL. 7 



ing degree of plasticity toward the center without a positive de- 

 marcation of the danger zone. Upon this treacherous surface 

 a mammal would be unaware of danger until the dust-covered 

 surface yielded under his weight. His sudden start or his leap 

 for safety would make all the more complete his entanglement. 



While these exposed traps must have been in many cases pas- 

 sive, concealed in an open or perhaps but slightly wooded 

 locality where animals would blunder into them, they must often 

 also have been actively attractive to animals through the two 

 important factors of water and food. During a considerable 

 period of time spent in working these fascinating deposits, the 

 author has had frequent recourse to the water accumulated in 

 depressions of the asphalt. This water has proven quite accept- 

 able for drinking and for bathing. As algae accumulate, frogs, 

 toads, dragonflies, mosquitoes, and other insect forms invade 

 it; rushes and marsh-grass border the pools, their roots actually 

 in contact with asphalt of the highest degree of tenacity. In 

 a number of cases the asphalt accumulations represent depres- 

 sions in the general surface of the country where not only the 

 direct rainfall would be temporarily held empounded but more 

 lasting pools representing surface drainage or even seepage 

 would accumulate. The presence of bedded leaf-masses and of 

 water-worn fragments of wood intermingled with the animal 

 remains would support the view that there were at times ponds 

 of a more or less permanent nature. The animals of poorly 

 watered regions in the southwest are perforce far from fas- 

 tidious in the matter of drinking water; hence the herbivorous 

 mammal must certainly have found the vicinity of these water 

 pools one offering very positive attraction as to water and 

 perhaps grass as well. 



The entanglement of one ungulate would suffice to attract 

 a multitude of carnivores. The creature probably acted not 

 infrequently as live bait for a considerable time, so that its 

 struggles and outcries served to whet the appetites and overcome 

 the instincts of caution in the hungry carnivore. It appears 

 from Merriam's studies that young animals or else old or dis- 

 eased individuals have very frequently been thus tempted, 

 though there appear animals of all ages. 



