212  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1903.  [bull. 225 
some  .salt  of  cadmium.     These  are  weathered  from  blende  which  car- 
ries a  small  percentage  of  cadmium. 
The  extent  of 'the  deposits. — The  ores  of  zinc  are  known  to  extend 
as  already  stated,  over  a  distance  of  about  40  miles.     They  are  nol 
continuous  throughout  that  distance,  of  course,  but  are  found  in  large 
quantity  at  many  places  and  have  been  traced  by  the  float  at  small  inter 
vals  over  much  of  the  intervening  space.     The   largest   body  of  ore 
exposed  is  in  the  old  mine  at  Jefferson  City,  where  open  cuts  have 
developed  it  along  a  north-and-south  line  for  about  600  feet,  with  an 
average  width  of  75  feet  and  depth  of  20  to  40  feet.     This  is  by  nc 
means  the  total  extent  of  the  deposit  at  that  place,  and  its  continua 
tion  toward  the  northeast  is  assured  by  the  presence  of  the  float  ore 
The  silicates,  in  particular,  are  a  ready  means  of  tracing  the  zinc 
deposits,  for  they  persist  in  the  residual  clays  long  after  the  disinte-i 
gration  of  other  materials.     The  slopes  of  the  limestone  hills  are  gentle, 
and  the  float  would  not  be  likely  to  travel  far  from  its  source.     Thus 
its  wide  distribution  is  a  guarantee  of  an  equally  wide  extent  of  the 
ore.     It  is,  accordingly,  probable  that  the  developments  have  shown 
up  only  a  very  small  fraction  of  the  ore  available. 
Sourer  of  the  material. — The  origin  of  the  blende  is  a  subject  on 
which  there  is  little  evidence.  The  inclosing  rocks,  so  far  as  known,i 
contain  no  trace  of  zinc,  nor  is  it  known  in  any  of  the  other  formations 
which  come  to  the  surface  in  the  Great  Valley.  The  secondaiy  blende 
and  calcite  appea.r  to  replace  the  finer  fragments  of  the  original  lime- 
stone, the  blende  being  substituted  for  a  certain  amount  of  the  lime- 
stone, and  the  limestone  recrystallizing  as  calcite.  The  ore  and  gangue 
were  clearly  deposited  from  aqueous  solutions.  From  the  entire 
absence  of  metamorphism  in  the  interior  of  the  limestone  fragments,  it 
can  be  inferred  that  the  solutions  wrere  either  only  slightly  heated  or 
cold.  In  regions  where  metamorphism  is  due  to  heat,  its  results  are 
found  unifortnty  in  all  portions  of  the  rock  mass  affected.  Nor  are 
there  any  bodies  of  igneous  rock  cutting  the  limestones  at  the  surface 
in  this  region,  from  which  the  heat  might  have  been  derived.  It  is, 
of  course,  possible  that  such  may  exist  beneath  the  surface,  but  that 
would  be  a  pure  assumption. 
Heated  waters  are  found  emerging  from  the  limestones  in  a  fewi 
places  in  the  Appalachian  uplift  at  the  present  day,  so  that  it  is  pos- 
sible that  the}^  may  have  issued  in  this  region  at  some  previous  time. 
The  present  heated  waters,  however,  do  not  contain  zinc.  Moreover, 
the  ability  of  cold  waters  to  break  up  and  replace  calcite  and  blende  is 
amply  shown  by  the  alterations  of  these  minerals  into  carbonates  and 
silicates  in  all  of  their  weathered  outcrops.  The  surface  waters  which 
effect  this  change  are  probably  slightly  charged  with  carbonic  acid 
from  vegetable  matter. 
The  only  source  for  the  blende  which  appears  possible  is  some  porJ 
