486  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1906,  PART    T. 
The  Tushar  Mountains,  near  the  north  end  of  which  the  sulphur 
beds  occur,  are  composed  of  eruptive  rock  consisting,  so  far  as 
examined,  of  flows,  breccias,  and  tuffs  of  rhyolite  overlain  in  places 
by  andesite.  Much  of  the  tuff  is  soft,  fine-grained,  and  snowy  white. 
The  rhyolites  presumably  rest  upon  sedimentary  rocks,  as  in  neigh- 
boring regions,  and  are  apparently  several  thousand  feet  thick.  The 
sulphur  occurs  in  the  white  tuff,  which  some  of  the  miners  call 
"  gypsum." 
In  the  vicinity  of  Sulphurdale  basalt  overlies  the  rhyolites  and 
andesites  in  slightly  eroded  flow  sheets  and  crater  cones.  Two  con- 
spicuous cones  having  well-defined  craters  occur  near  the  sulphur 
beds,  one  about  10  miles  to  the  southwest,  the  other  3  miles  west  of 
the  camp.  Still  other  cones  and  associated  Hows  are  situated  farther 
north,  forming  a  linear  group  lying  essentially  parallel  to  the  neigh- 
boring mountain  ranges.  The  fresh  appearance  of  the  lavas  and  the 
slight  amount  of  erosion  of  the  cones  indicate  recent  formation,  and 
lavas  of  similar  character  near  Blackrock  rest  upon  the  Bonneville 
beds,  from  which  fact  it  seems  probable  that  the  cones  were  formed 
during  late  Quaternary  time. 
Structure. — There  are  reasons  for  believing  that  the  sulphur  beds 
are  located  in  or  near  a  zone  of  intense  faulting  and  volcanic  activity 
and  thai  the  conflicting  forces  causing  the  disturbances  have  not  yet 
reached  a  state  of  equilibrium.  Evidence  of  this  is  found  not  only  in 
the  vicinity  of  Sulphurdale,  but  for  long  distances  both  north  and 
south  of  this  place. 
Along  the  western  border  of  the  high  plateaus  of  Utah,  from  St. 
George  northeastward,  several  basins  have  been  formed  by  faulting, 
accompanied  or  followed  by  the  movement  of  large  crust  blocks.  Rush 
Lake  Valley  and  Parowan  Valley  are  perhaps  the  most  conspicuous  of 
these  basins.  The  st  rata  of  t  he  plateau  to  the  east  lie  essentially  hori- 
zontal, while  tliuM'  to  the  wesl  dip  eastward  beneath  the  valleys.  In 
the  Beaver  basin  this  simple  relation  is  complicated  by  the  great 
masses  of  effusive  rock  which  cover  the  sediments,  but  west  of  the 
basin,  in  the  vicinity  of  Minersville,  and  again  at  the  north  end  of 
Mineral  Range,  the  Paleozoic  sediments  appear  underneath  the  effu- 
sives,  dipping  eastward  beneath  the  basin.  From  this  it  is  inferred 
that  Beaver  Valley,  like  those  farther  south, is  due  tocrustal  movement 
and  that  the  great  fault  zone  which  follows  the  western  margin  of  the 
plateau  through  Rush  Lake  and  Parowan  valleys,  and  along  which 
displacements  of  thousands  of  feet  are  known  to  have  occurred,  prob- 
ably continues  through  Beaver  Valley,  and  thence  northeastward 
past  Sulphurdale. 
Several  phenomena  noted  mark  this  zone  as  one  of  recent  disturb- 
ance. The  line  of  sulphur  beds,  from  which  hydrogen  sulphide  is  still 
escaping  in  large  cpuantities,  and  less  exactly  the  line  of  the  recent 
