96 WALKS AND TALKS. 



cut channels across the ancient ones, and lava-topped intervals 

 remain. These are table mountains. Further south, in Tuo- 

 lumne county, the ancient and the modern drainage both 

 moved from north to south. The aucient channels, therefore, 

 stretched north and south, and the lava-sheets which filled 

 them stretched from north to south. The modern water- 

 courses have shunned the hard lava, and have dug their chan- 

 nels alongside of the lava, in less consolidated materials- 

 gravels or slates. These positions were formerly the elevated 

 banks of the streams. Thus, the undisturbed, elongated lava- 

 sheets, which rested on the bottoms of the ancient channels, 

 now rest on elongated ridges. The ancient bottoms are be- 

 neath these tables. Over the ancient bottoms were distrib- 

 uted the auriferous gravels from the mountains ; and here they 

 are still found. They are the "deep placers;" and are ex- 

 plored by drifting in from the sides. The beds of the modern 

 streams, strewn with auriferous sands from the same sources 

 form the so-called "shallow placers." 



- XVII. I M PRISON ED HEDA.T. 

 INTERNAL CONDITION OP THE EARTH. 



IT is a startling circumstance to see warm water issue from 

 a hole bored in the rocks. The common impression is that 

 deep waters are cooler than the average temperature at the 

 surface. A well of this sort is styled Artesian; and you will 

 understand that the water is forced up by pressure of other 

 water standing somewhere, at a higher level, and freely com- 

 municating with this. You have learned, for instance (Talk 

 XIII), that strata often dip down from their place of outcrop 

 to a great depth into the earth. Suppose a porous formation, 

 like a sandstone, thus goes down from the surface ; the rain 

 which falls on its outcrop must soak into the rock and saturate 

 it. This is then a water-bearing stratum. In descending ob- 

 liquely it passes under many places ; and if a hole should be 

 bored from the surface to this stratum, the water would rise 



