520 University of California Publications in Geology [Vol. 10 



imperial elephant, early types characteristic of Rancho La Brea. 

 Equus pacificus, the common horse of Fossil Lake, is apparently a 

 more specialized form than E. occidentalds of Rancho La Brea, and 

 the prong-horn Antilocapra represented at Fossil Lake is apparently 

 a specialized descendant derived from a stock near the Rancho La 

 Brea Capromeryx. It may be that the Fossil Lake assemblage is not 

 older than that of Rancho La Brea, but is merely a contemporaneous 

 northern phase of the same fauna. 



The absence of bison from the Fossil Lake fauna may be sufficient 

 warrant for considering it older than the upper Lahontan stage, but 

 this item may not be of greater value in separating the time of depo- 

 sition of the two sets of deposits than is the physical evidence used 

 by Gilbert to prove their contemporaneity. 



If Gilbert is correct in his correlation of Fossil Lake with Lahontan 

 and Bonneville, the Lahontan fauna belongs to a time separated from 

 the present by a period of widespread extinction of organic types and 

 by evolution or introduction by migration of an almost entirely new 

 mammal series, as two-thirds of the Fossil Lake mammal species are 

 extinct and the stage as a whole does not appear like the latest 

 Pleistocene. 



In some respect the closest resemblance of the few mammal frag- 

 ments from the Lahontan is to the fauna of Rancho La Brea. Of 

 the known Lahontan fauna — Elephas columbif, Equus near pacificus, 

 Equus near occielentalis, Bison, sp., Camelops, sp., Felis atrox — all of 

 the forms so far as known, with the exception of the horse near E. 

 pacificus, are found at Rancho La Brea. It seems possible that the 

 Lahontan fauna may be representative of a period including the Fossil 

 Lake and Rancho La Brea stages. On this basis it would appear that, 

 whatever the more exact correlation, the Lahontan stage represents a 

 time when the life of this region and that of North America in general 

 was made up of mammal types of which from 60 per cent to 70 per cent 

 have since become extinct and are replaced by new types. 



We are not of course able to make an exact correlation for the 

 whole of North America in Pleistocene time, but we know that many 

 of the types included in the Rancho La Brea and Fossil Lake assem- 

 blages had a very wide geographic distribution, and it seems that a 

 change such as has occurred since Rancho La Brea and Fossil Lake 

 time is typical of a faunal change over a large part of America. 



That an extinction of 66 per cent of the mammals in any fauna 

 might occur in a very short time one might readily grant, but the 



