76 University of California Publications in Geology (Vou. 7 
ing degree of plasticity toward the center without a positive de- 
mareation 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 outeries 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. 
