50 



University of California Publications. 



[Geology 



tected the Mesa Basalt. It seems improbable that the Mesa 

 Basalt has been covered by considerable deposits of any kind, 

 as the large level stretches now exposed appear to be entirely 

 bare. 



There is strong evidence against the suggestion that the 

 Thousand Creek Beds represent an accumulation of the latest 

 wash from Thousand Creek and other similar drainage, as this 

 would increase the length of the period back to the initiation of 

 the first cutting through the Mesa Basalt. The strongest argu- 

 ment in favor of relatively late age of the Thousand Creek Beds 

 is obtained by Heindl's study of the Railroad Ridge lava, and 

 by his discovery of a basalt pebble in the gravel -immediately 

 under the lava. The basalt pebble from the gravels below the 

 lava is considerably decomposed, but seems to be rather nearer 

 the type of the Mesa Basalt than it is to that of the older Pueblo 

 Range lavas. Heindl has also called attention to the fact 'that 

 the Railroad Ridge lava is not broken up as it might be if it- 

 were a block which had been dropped a considerable distance. 

 In the absence of well preserved material, the basalt pebble from 

 below the lava is hardly sufficient evidence to prove that the 

 Railroad Ridge lava is a flow of later age than the Mesa Basalt. 

 The lack of disturbance of the Railroad Ridge lava does not 

 necessarily indicate that this block has not been moved, though 

 minor disturbance might naturally be expected. 



Judging from the evidence of the fauna, it seems probable 

 that the Thousand Creek exposures represent in the main a 

 single period of deposition. Upon the lower terraces border- 

 ing the valley there may be Pleistocene deposits with a fauna 

 containing Equus. Such deposits, if they occur, are apparently 

 not thick, and their presence would hardly confuse the problem 

 as to the age of the great extent of exposures with a late 

 Tertiary fauna. 



It is very desirable that more evidence be obtained relative 

 to the purely geologic history of the beds at Thousand Creek. A 

 determination of the exact geologic position of these beds may 

 depend finally upon a study of the fauna, but from the standpoint 

 of the palaeontologist it is most desirable to have the evidence of 

 sequence of faunas based upon stratigraphic succession. 



