keith.]  RECENT    ZINC    MINING    IN    EAST    TENNESSEE.  213 
tion  of  the  earth's  crust  which  lies  below  the  sedimentary  rocks.  As 
nearly  as  can  be  inferred,  the  igneous  rocks  which  occupy  this  position 
are  about  15,000  feet  below  the  surface.  To  support  the  conclusion 
that  this  is  the  actual  source  of  the  zinc,  there  is  only  the  negative  evi- 
dence before  mentioned;  nor  is  there  any  known  reason  why  the  zinc 
should  be  deposited  along  this  line  rather  than  along  others  in  similar 
situations. 
In  this  zinc  belt  the  zinc  was  deposited  only  in  the  upper  portions  of 
the  Knox  dolomite.  However,  this  is  not  the  only  part  of  the  forma- 
tion which  contains  zinc,  but  it  is  found  in  the  bottom  of  the  Knox 
dolomite  at  a  number  of  localities  about  30  miles  north  of  Knoxville. 
Nor  is  the  zinc  limited  in  any  of  these  general  localities  precisely  to 
:  one  horizon,  but  it  varies  more  or  less.  This  would  be  expected  from 
waters  which  circulate  through  masses  of  broken  rock.  Judging  from 
similar  phenomena  elsewhere,  the  zones  of  crushing  and  breaking  dip 
beneath  the  surface  at  considerable  angles  and  extend  to  great  depths. 
It  is  therefore  probable  that  here  the  zinc-bearing  waters  rose  from 
below  through  the  mass  of  broken  limestone,  dissolving  here  and 
depositing  there,  as  local  conditions  permitted. 
