MODE OF ACCUMULATION OF THE FOSSIL REMAINS 



The accumulation of the remains emhedded in the fossil beds at Rancho La 

 Brea, as also their assembling in the greater number of the asphalt deposits of 

 southern California, has evidently been controlled by conditions differing 

 largely from those which have produced other kinds of fossil deposits. In 

 the excavation work which has been carried on at Rancho La Brea up to the 

 present time, though a considerable variety of deposits has been encountered, 

 the fossil bones are found to be almost entirely confined to beds which are either 

 pure asphalt or consist of some other material impregnated with tar. When 

 pure sand or clay is present without a tar impregnation there is a conspicuous 

 absence of skeletal remains. While it is possible that some of the impreg- 

 nated strata may have received their bituminous content since the bones were 

 deposited, the facts of occurrence seem to indicate quite clearly that the presence 

 of tar is the condition which has controlled the accumulation of skeletal remains. 



The manner in which tar or asphalt pools may catch unsuspecting 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 been frequently so firmly entrapped that they died 

 before being discovered, or if found alive were extricated only with the greatest 

 difficulty. | As seen at this locality, the tar issuing from springs or from seep- 

 ages 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 evaporation of the lighter constituents 

 until it becomes as solid as an asphalt pavement. Between the hard and the 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, t 



Judging from reports of the earliest observers who examined the tar seeps 

 at Rancho La Brea, some of the pools were many feet in diameter in their 

 natural state, and might easily in the course of years have caught a great 

 number of even the largest mammals. 



In the natural accumulation of remains at the tar pools through accidental 

 entangling of animals of all kinds, it is to be presumed that a relatively large 



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