352 WALTER C. MEN DEN HALL 



itself was shelving and indefinite, but its general position is well 

 marked by the molluscan remains. 



Beyond and below the gravel zone is one of sand, and still farther 

 east beyond this is the silt-covered bottom of the old lake Cahuilla. 

 The deposits here are impalpably fine, laminated clays which, when 

 stirred, as in a much-traveled road, become a tawny flour and when 

 moistened are transformed into a smooth, adhesive mud. 



RESUME OF GEOLOGIC HISTORY 



The story of the development of this part of the country cannot 

 be read with any approach to accuracy as yet for any period beyond 

 the Miocene. The rocks which represent earher time are mar- 

 morized limestones, schists, and gneisses as to whose age there is 

 much doubt. The slight existing evidence points toward the Car- 

 boniferous as the period during which the limestones were deposited 

 here. Whatever their age, their condition now indicates that their 

 history previous to the Miocene was one involving deep burial and 

 intense earth strain. They were upturned, intruded, and crystallized, 

 uphfted and eroded into a mountainous topography, and at the 

 beginning of the late Miocene formed islands in a sea teeming with 

 life. Volcanic forces were active at this time and the flanks of the 

 old land mass are partly buried under the effusive material which 

 issued then, and the muds and the littoral whose fragments were 

 supplied from volcanic sources are conspicuous at many points. 

 But as the period advanced, vulcanism ceased and the present Car- 

 rizo and Black mountains were surrounded and perhaps for a part 

 of the time were submerged beneath a clear sea in which the myriad 

 forms of the life of the period swarmed. Still later in the Miocene 

 the character of the sea changed. Instead of clear, salt water, some 

 re-alignment of forces caused great quantities of muddy brackish 

 water to spread about the old islands. Oysters of many forms, 

 some of them of great size, some very tiny indeed, flourished. The 

 heavy silts of these muddy waters accumulated to great depths as 

 the land subsided. Finally the waters withdrew, presumably because 

 of re-elevation, and the region was land again as it had been before, 

 and the shells of many of the creatures which had Hved in the clear 

 and then in the muddy waters were preserved in the accumulated 



