EXTINCT FAUNAS OF THE MOHAVE DESERT 253 



of the Barstow section may be considerably older than Upper Miocene 

 and may constitulc a distinct formation. The name Eicardo formation 

 is nsed for the strata with a Pliocene fauna at Eicardo. 



The youngest fossil beds of the Mohave region appear in a small 

 basin about 20 to 40 miles east of Barstow. The deposits cover an 

 area about 35 miles in lengih and represent accumulation in a small 

 body of fresh-water, to which j\Ir. Buwalda has given the name Manix 

 Lake. The deposits consist of clays and sands aggregating about 75 

 feet in thickness. Their accumulation was initiated by the raising of 

 a barrier across the Mohave Eiver drainage, causing the ponding of the 

 river which formed Manix Lake. The lake disappeared and deposition 

 ceased when the river cut through the barrier across its path. 



Eemains of extinct vertebrates are found over a wide area in the 

 deposits of the Mohave region. They are not abundant in many places, 

 and one may search long for even a fragment of a bone or tooth. In a 

 few localities fragmentary specimens were found scattered over the 

 ground in considerable numbers, but connected parts of skeletons are 

 rare. At several points where bones were found well exposed, and in 

 their original position in the rock, they seemed to be scattered and dis- 

 connected, showing that the parts of skeletons were generally widely 

 separated and broken or weathered before final burial. The process of 

 entombment was probably similar to that in operation on the desert at 

 the present time, where bones of horses and cattle are pulled apart by 

 coyotes, scattered by rain-wash, and in a large measure rotted away 

 before any portion of the animal is permanently covered over. 



The collections obtained include several thousand specimens, mostly 

 teeth and portions of limb-bones. In a few cases, good jaws and parts 

 of skulls were secured, but unlike the occurrence in many of the forma- 

 tions in the west, these beds seem almost never to contain complete 

 skeletons. 



In the Miocene beds of Barstow vertebrate remains are found almost 

 exclusively in the uppermost zone. In the Eicardo Pliocene fossil re- 

 mains were found in several parts of the section, but the best represen- 

 tation of the fauna appears near the middle and toward the top of the 

 formation. 



Although only a few localities have been found at which even small 

 collections of mammalian bones can be obtained in the area of the 

 Mohave region examined, it is evident that deposits representing the 

 formations in which bones occur are very widely spread over this area, 

 and future exploration may be expected to add greatly to the informa- 

 tion now available. 



The formations containing mammalian faunas in the Mohave area, 

 and their approximate relations to the recognized geological scale are 

 as follows : 



