188  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1903.  [bull. 225. 
cut  through  by  Millstone  River,  the  principal  tributary  of  the  Rari- 
tan,  and  excellent  exposures  are  seen  near  the  village  of  Rocky  Hill. 
The  copper  property  is  about  3  miles  north  of  this  town,  on  the  open, 
smooth,  cultivated  fields  on  the  east  side  of  the  river,  at  an  elevation 
of  about  200  feet  above  the  sea. 
Geology. — But  two  rock  types  are  found  in  the  vicinity,  trap  (or 
diabase)  and  red  shales  of  the  Newark  formation.  The  trap  is  a  coarse- 
grained, bluish-gray  rock  of  very  uniform  character.  It  forms  a  thick, 
gently  inclined  sheet  lying  conformable  with  the  red-shale  beds  into 
which  it  was  intruded.  These  shales  are  always  more  or  less  highly 
altered  near  the  igneous  rock,  sometimes  for  a  distance  of  100  feet 
vertically  above  the  sheet.  This  alteration  is  particularly  marked  at 
the  copper  mine. 
The  copper  property. — The  copper  mine  is  located  about  1  miles 
from  Rocky  Hill  Station,  on  the  east  side  of  Millstone  River  and  150 
feet  or  so  above  its  bed.  A  wooded  area  with  an  outcrop  of  trap 
forms  the  summit  of  the  hill,  the  mine  being  located  on  the  smooth, 
rounded  slopes  below,  that  extend  downward  to  the  Raritan  Canal. 
The  property  was  worked  early  in  the  last  century,  and  was  cleaned 
out  and  reopened  some  years  ago  by  the  present  owners,  but  no  cop- 
per has  been  produced  lately.  The  workings  consist  of  several  verti- 
cal shafts,  now  caved  in,  a  drain  tunnel  1,200  feet  long,  and  a  new 
incline  shaft,  together  with  stoped-out  chambers.  The  main  vertical 
shaft  is  said  to  be  150  feet  deep,  but  is  tilled  with  water  to  the  level 
of  the  old  drain  tunnel,  and  the  workings  at  present  accessible  are  all 
above  this  point. 
The  deposit  consists  of  an  ore  seam  varying  from  an  inch  to  a  foot 
or  more  thick,  occurring  in  and  conformable  to  altered  shales.  These 
rocks  are  shales  altered  by  contact  metamorphism,  resulting  from  the 
intrusive  sheet  of  trap.  They  are  hard,  dense,  and  have  lost  their 
fissile  character,  and  are  more  properly  called  hornstones.  A  notice- 
able feature  is  a  peculiar  spotting  due  to  the  presence  of  dark-colored 
spherical  segregations,  which  are  scattered  singty  or  in  groups  through 
the  dark-purple  rock.  These  vary  from  minute  pellets  up  to  globes 
an  inch  across,  but  are  commonly  about  one-fourth  inch  in  diameter. 
They  consist  of  green  hornblende  in  part  altered  to  chlorite,  and  are 
commonly  surrounded  by  a  rim  1  millimeter  wide  of  bleached  rock. 
This  is  the  rock  which  carries  the  ore,  but  when  mineralized  the 
groundmass  is  rotted  and  altered  to  a  pale  flesh-colored  or  white, 
kaolin-like  mass,  in  which  the  dark-green  spherules  resemble  plums  in 
a  pudding. 
The  ores. — The  ores  consist  of  native  copper,  red  oxide  of  copper 
(cuprite),  black,  glassy -looking  tenorite  (black  oxide  of  copper),  mala- 
chite, chrysocolla,  copper  glance,  and  rarely  bornite.     The  ore  seam 
