Introduction to Life of the Lakes. in 



of this older lake period, the period of quartz 

 filled cavities. This fact of quartz infiltration 

 is also observed in some belts of Miocene rock 

 of the Willamette valley in which the cavities 

 of fossil clams contain perfect casts of chal- 

 cedony or agate. In both of these localities 

 the liquefaction of the infiltrated silica seems 

 due to the pressure and heat generated under 

 the weight of later deposits of sediment, since 

 removed by the wash and wear of atmos- 

 pheric erosion. The silica infiltration oc- 

 curred after the inclosure of the specimen by 

 the sediment. The shell drifts into deep water 

 and sinks into- the soft oozy mud of the bot- 

 tom. As long as there remains any crack or 

 chink of the shell accessible to this mud, the 

 weight of the water presses in more. In those 

 shells that are thus entirely filled the process 

 of filling up the inner cast is completed. But 

 if through any cause the shell is completely 

 enveloped by the sediment before the inner 

 cavity is filled, then this upper and unfilled 

 portion remains empty. Now it is this rem- 

 nant of space that is filled with silica. The 



