88  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1905. 
cyanide  is  finally  caught.  In  the  mill  now  under  construction  at  an  adjacent  mine  by  the 
Sevier  Consolidated  Gold  Mining  Company  a  somewhat  different  plan  is  adopted,  including 
concentrating  on  tables,  as  well  as  plate  amalgamation  and  eyaniding. 
Geology. — About  100  miles  south  of  Salt  Lake  City  the  railroad  enters  the  characteristic 
Plateau  Province,  where  the  strata  lie  horizontal  and  little  evidence  of  volcanic  action  is 
seen.  For  70  miles  the  road  ascends  the  broad  Sevier  Valley  to  its  terminus  at  Marysvale. 
Cliffs  of  light-colored  reddish  or  white  1><m1s  belonging  to  the  Tertiary  form  a  conspicuous 
wall  on  the  west  side,  while  to  the  east  rises  the  broad  Wasatch  plateau,  capped  by  rocks  of 
the  same  age.  Near  the  town  of  Richland  darker  volcanic  masses  begin  to  appear  on  both 
sides  of  the  valley:  their  character  as  flows  covering  the  Tertiary  rocks  is  clearly  indicated. 
This  region  has  been  described  in  considerable  detail  by  Major  Dutton  in  his  report  on  the 
High  Plateaus  of  Utah,  a  He  shows  on  the  west  side  of  the  upper  Sevier  Valley  a  large 
area  of  rhyolite  culminating  in  the  peaks  of  Belknap  and  Delano,  which  practically  over- 
look Marysvale.  On  the  east  side  of  the  valley  the  heavy  volcanic  masses  consist,  accordii 
to  Major  Dutton,  of  a  trachytic  rock. 
At  the  little  station  of  Sevier  (elevation,  5,557  feet),  a  few  miles  north  of  Marysvale,  the 
valley  contracts,  and  for  a  short  distance  the  river  flows  through  a  deeply  cut  canyon  in  the 
rhyolitic  rocks,  evidently  excavated  since  the  eruption  of  the  rhyolites,  which  appear  te 
have  formed  a  temporary  dam  across  the  valley.  The  area  indicated  on  Dutton's  map! 
as  rhyolite  covers  at  least  1,500  square  miles.  Over  the  larger  part — that  to  the  north  of 
the  higher  peaks — the  rocks  are  tuffaceous  and  more  or  less  clearly  water-laid.  The  wagon 
road  from  Sevier  to  the  Annie  Laurie  mine  follows  up  Clear  Creek  in  a  westerly  direction 
between  dill's  of  well-bedded,  light  rhyolite  tull's.  Eight  miles  from  Sevier  it  turns  south- 
ward into  the  canyon  of  Mill  Creek  and  from  that  point  rapidly  ascends  toward  the  higher 
central  peaks  in  the  range,  at  the  northern  foot  of  which  the  mine  is  located.  We'll  up 
toward  the  summits  the  tuff  is  replaced  by  massive,  light-colored  rhyolite,  easily  breaking 
into  small  fragments,  which  form  long  white  talus  slopes  along  the  sides  of  the  ridges. 
Broad  views  open  toward  the  north,  where  the  rhyolite  tuffs  extend  for  more  than  25  miles 
as  a  monotonous,  high,  arid  plateau,  dotted  here  and  there  with  a  scant  vegetation  of 
junipers.  From  points  a  short  distance  north  of  the  mine  long  and  broad  but  gentle  slopes 
begin,  partly  covered  with  bunches  <>!'  lir  and  dotted  in  the  autumn  with  brilliant  foliage 
of  aspens  and  low'  bushes.  These  long  slopes  extend  up  from  the  plateau  level  tow  aid  the 
highest  peaks,  of  which  only  one,  called  Bare  Mountain,  is  visible  from  this  point.  A 
number  of  them  exceed  12,000  feet  in  elevation  and  are  among  the  highest  in  Utah.  The 
rhyolite  continues  almost  up  to  the  mine,  which  is  situated  at  the  point  where  Mill  Creek 
begins  to  expand  into  a  glacial  amphitheater  and  where  the  little  mining  town  of  Kimbcrly 
has  obtained  a  precarious  foothold  on  one  of  the  steep  rhyolite  slopes.  The  upper  basin  of 
Mill  Creek  is  occupied  by  a  massive  greenish-gray  rock,  much  chloritized,  and  showing  small 
crystals  of  feldspar.  This  is  the  country  rock  in  which  the  deposits  occur.  Microscopical 
examination  shows  it  to  contain  a  considerable  amount  of  quartz  in  the  groundmass,  while 
the  phenocrysts  arc  in  part  alkali  feldspar  and  in  part  moderately  basic  soda-lime  feldspar. 
There  are  also  small  foils  of  biotite,  usually  more  or  less  completely  converted  into  chlorite. 
On  the  whole,  the  rock  should  probably  be  classified  as  a  dacite.  This  area  probably 
extends  southeastward  across  the  summit  of  the  range  toward  Marysvale.  Near  the 
Sevier  mine  and  on  the  ridge  between  the  Sevier  and  Annie  Laurie  dikes  of  dioritic  or  mon- 
zonitic  rocks  appear.  These  are  dark  green  and  contain  no  quartz.  One  of  these  dikes 
outcrops  close  to  the  Sevier  mine,  but  it  is  not  evident  that  any  close  connection  exists 
between  the  dike  and  the  deposit.     The  veins  are  wholly  in  the  dacite. 
From  the  summit  a  sharp  descent  leads  dowm  toward  Marysvale,  and  in  the  lower  part  of 
Bullion  Creek,  according  to  Dutton,  outcrops  of  a  quartzite  occur,  which  are  referred  by 
him  to  the  Jurassic. 
Ore  deposits. — The  deposits  occur  in  well-defined  quartz  veins  cutting  through  the  core  of 
this  old  volcanic  district.     As  far  as  knowTn,  no  deposits  occur  in  (he  rhyolite  or  in  the 
a  U.  S.  Geog.  and  Gcol.  Surv.  Rocky  Mt.  Region,  1880. 
