EXTINCT FAUNAS OF THE MOHAVE DESERT 263 



At the present time we are not in a position to state definitely the 

 exact position or relationship of the Manix fauna with relation to other 

 Pleistocene life in the west. The problem of the Pleistocene in this 

 region is complicated and large, and the many elements still require 

 much study before their interrelations can be determined. The Rancho 

 La Brea fauna seems to contain elements similar to those of Manix, 

 whether it is older or younger is not yet entirely clear. 



The significance of the Manix fauna in relation to its environment 

 is unfortunately not large. The presence of camels, horses, and ante- 

 lopes indicates a climate somewhat more humid than that in this 

 region at the present time, and such was the suggestion furnished by 

 Mr. Buwalda's work on the physical history of the Manix Lake basin. 



Significance of the Faunal Succession in the Mohave 



The physical history of the Mohave area, in the time that has 

 passed since the accumulation of the oldest formation containing a 

 mammalian fauna in this basin, is only a small part of the long and 

 complicated geologic story of the region; but the changes that have 

 occurred since the earliest of these records of life were completed take 

 on stupendous proportions when measured against human standards 

 of stability. Since the deposition of the oldest beds of the Barstow 

 section, not less than 8,000 ft. of known sediments have been laid 

 down in this region, and there are evidences of long periods from which 

 the only record that we have is of erosion instead of deposition. The 

 strata of both the Barstow and Ricardo sections have been subjected to 

 extreme movements of the earth's crust in folding and faulting or break- 

 ing. They have also been extensively eroded or worn down, and the 

 strata now exposed can be considered only as remnants of the original 

 mass. In terms of accumulation and erosion of deposits, judged by 

 the best estimates that we can make, the lapse of time since burial of 

 the oldest mammal remains in this region must be very long. 



Physical changes of great significance in the history of this region, 

 and of the life in it, are also noted in variations in the nature of the 

 bordering mountain ranges. At present the Mohave owes its distinc- 

 tive characters in large measure to separation from the Pacific coast by 

 high ridges to the west. Throughout a large part of the known life 

 history of this region, a barrier seems to have existed between the Mo- 

 have area and the Pacific coast province. The height of the separating 

 wall has presumably varied much, being relatively small in Miocene 

 time, and probably reaching its maximum since the Ricardo Pliocene. 

 Variation in height of the barrier depended on the balance between 

 erosion constantly wearing it down, and on the magnitude of crustal 

 movements concerned in the making of the mountain chains. To some 



