BMMons.]  CACTUS  COPPER  MINE,  UTAH.  245 
ical  sketch  gives  an  accurate  reproduction  of  the  general  outlines  of 
a  portion  of  the  range  and  the  relative  position  of  the  two  mines  and 
their  dependent  towns  of  Frisco  and  New  house." 
The  range  is  divided  by  saddles,  which  evidently  have  a  structural 
signification,  into  three  massifs  or  orographic  blocks.  The  southern, 
known  as  the  Grampian  Hills,  which  lies  opposite  the  Horn  Silver 
mine,  is  composed  mainly  of  bedded  blue  limestone.  Along  the  east 
base  of  this  massif  runs  a  north-south  fault,  separating  the  limestones 
from  recent  andesitic  rocks  which  fill  the  valley  in  which  lies  the 
town  of  Frisco,  and  form  low,  broken  hills  extending  out  eastward 
from  the  San  Francisco  Range  to  the  north  of  that  town.  It  is  within 
the  fault  fissure,  which  at  that  point  is  over  100  feet  wide,  that  have 
been  found  the  important  ore  bodies  of  the  Horn  Silver  mine.  In 
this  mine  the  Horn  Silver  fault  appears  to  have  a  trend  a  little  east 
of  north,  but,  according  to  Mr.  W.  A.  Hooker,  the  mining  engineer 
who  made  a  detailed  study  of  this  mine  in  1879,  the  average  direction 
in  the  2  miles  over  which  it  has  been  traced  is  N.  10°  W. 
The  saddle  which  separates  the  Grampian  Hills  from  the  central 
massif  is  the  locus  of  an  east-west  fault,  which  branches  off  at  right 
angles  from  the  Horn  Silver  fault  and  which  brings  up  monzonite  (a 
granitic  rock  intermediate  in  mineralogical  composition  between 
granite  and  diorite)  on  the  north  side  into  juxtaposition  with  lime- 
stone on  the  south.  The  central  massif  extends  north  as  far  as  the 
depression  of  Copper  Gulch,  which  cuts  diagonally  across  the  range 
in  a  northwesterly  direction.  The  higher  portion  of  the  massif  is 
apparently  composed  of  limestones  overlain  by  quartzites.  On  its 
southwestern  face,  as  described  in  the  above-mentioned  article,  is  a 
body  of  monzonite  which  has  intruded  an  extensively  altered  lime- 
stone, the  latter  being  changed  along  a  wide  contact  border  to  a  garnet 
•rock  abounding  in  the  lime-silicate  minerals  characteristic  of  contact 
metamorphism.  At  the  northern  point  of  this  massif  is  an  extensive 
outcrop  of  a  similar  granitic  rock,  having  apparently  a  somewhat 
more  dioritic  facies.  in  which  occur  the  deposits  of  the  Cactus  mine. 
rVhether  or  not  these  two  bodies  are  connected  at  the  surface  was  not 
determined,  but  they  evidently  form  part  of  the  same  batholith  of 
onzonite  which  has  intruded  and  altered  the  limestones. 
The  northern  massif,  which  extends  for  some  miles  beyond  the 
limits  of  the  map,  is  higher  than  either  of  the  others.  Its  culminat- 
ing point,  for  which  the  name  "  Golden  Horn  Peak  "  is  suggested, 
has  an  abruptly  escarped  face  to  the  southwest,  suggestive  of  fault- 
ng,  that  affords  an  excellent  section  of  the  sedimentary  rocks  of 
I     °  This  sketch  has  been  prepared  from  unfinished  plane-table  sheets  of  Mr.  F.  McLaugh- 
lin, topographer,  who  is  engaged  in  making  a  detailed  survey  of  the  district. 
