144  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1903.  [bull. 225. 
improbable  that  the  fine-grained,  granular  fades  (dloritic  type)  may 
represent  the  parent  mass  which,  on  breaking  into  the  overlying  sedi- 
ments  as  restricted  dikes,  cooled  in  the  porphyritic  facies  (porphyritic 
andesite),  or  breaking  upward  to  the  surface  flowed  out,  blanketing 
the  topography,  and  cooled  as  andesitic  breccia  (hornblende  andesite). 
Final  judgment  in  this  case  must  be  suspended  until  these  rocks  have 
been  thoroughly  studied  microscopically  and  chemically.  It  now 
seems  clear,  however,  that  at  least  one,  and  probably  two  periods  of 
intrusion  preceded  the  above  igneous  activity.  That  an  intimate  rela- 
tionship exists  between  these  intrusions  and  the  ore  bodies  which  have 
been  studied  thus  far  seems  indisputable.  The  determination  of  the 
nature  of  this  relationship,  however,  is  most  difficult.  Accordingly 
these  problems  assume  immense  practical  significance,  and  their  solu- 
tion may  reasonably  be  expected  to  throw  light  on  the  problems  of 
the  date,  origin,  and  occurrence  of  the  ore  deposits. 
Metamorphic  rod's. — The  entire  sedimentary  section,  embracing  all 
of  the  clastic  formations  represented  in  this  area,  has  been  traversed 
by  an  extensive  mass  of  the  dioritic  rock  and  penetrated  by  dikes  of 
andesitic  porphyry.  Adjacent  to  the  main  intrusive  body,  and  to 
some  of  the  minor  intrusions,  the  sediments  have  been  intense^  meta- 
morphosed. Numerous  contact-metamorphic  minerals,  such  as  garnet, 
epidote,  magnetite,  specularite,  serpentine,  spinel,  secondary  mica,  and 
others  have  been  developed,  and  the  lithologie  character  of  the  sedi- 
ments has  been  thoroughly  changed.  The  determination  of  the  pre- 
cise nature  of  the  metamorphism  by  tracing  strata  continuously  from 
their  unaltered  phases  through  transitions  into  their  altered  repre- 
sentatives is  attended  with  much  uncertaint}^,  owing  to  the  obliteration 
of  fossil  remains  by  metamorphism  and  to  the  presence  of  complex 
and  extensive  faulting  in  the  transition  zone  between  the  altered  and 
unaltered  sediments.  The  equivalency  of  certain  marbles,  quartzites, 
and  argillites  to  certain  limestones,  sandstones,  and  shales  has  been 
recognized.  Furthermore,  metamorphosed  sediments  have  been  noted 
in  localities  which,  though  relatively  remote  from  visible  intrusive 
masses,  are  in  the  neighborhood  of  strong  dynamic  movement. 
Geologic  structure. — The  sedimentary  formations  of  this  region  dip 
in  general  toward  the  northwest  about  30°  to  40°,  and  are  traversed  b}^ 
many  fissures,  faults,  and  intrusions.  Normally  the  oldest  formation 
therefore  outcrops  in  the  southeastern  portion  of  the  area,  while 
younger  formations  outcrop  successively  toward  the  northwest.  This 
general  structure,  however,  suffers  several  important  and  many  minor 
modifications  through  folding  and  faulting.  In  the  vicinity  of  Park 
City  proper,  the  general  northwesterly  dip  of  the  formations  grad- 
ually changes  to  a  northerly  and  northeasterly  dip,  until,  in  the  north- 
eastern portion  of  the  district,  the  dip  is  toward  the  east.  In  short,  the 
general  northwesterly  dip  gives  way  locally  to  a  broad  arch  along  a  NNE. 
