CHAPTER IX. 



LIFE OF UPPER LAKE REGION. 



TRANSITION. 



Before dismissing the history of this lower 

 or Miocene lake, let us ponder a moment on 

 the evidence we have of its long duration. 

 We have a sediment of mud three thousand 

 feet in vertical thickness, by far the greater 

 portions of very fine materials, indicating a 

 slow rate of deposit, other portions coarser, 

 indicating a more rapid deposit, but an aver- 

 age accumulation of not more than a small 

 fraction of an inch per year, and this built 

 up to the thickness of three thousand feet. 

 Yes, this work covered an immense period of 

 time. 



Now it was this vast accumulation of mud- 

 dy lake sediment that was slowly changed to 

 rock, every yard of it teeming with fragments 

 of organic forms that tell the story of its times. 



