SPURR    A 
GARRE 
^N'D]       GEORGETOWN     MINING    DISTRICT,    COLORADO.  103 
The  diorite  is  ordinarily  composed  of  black  or  greenish-black  horn- 
blende, white  to  gray  plagioclase  feldspar,  and  black  or  brownish 
biotite,  in  about  equal  proportions.  At  times,  however,  differentia- 
lion  processes  have  resulted  in  a  rock  composed  almost  entirely  of 
any  one  of  these  minerals.  Accessory  minerals  are  quartz,  epidote, 
calcite,  magnetite,  and  pyrite.  The  rock  usually  has  a  mottled  white 
and  black  appearance,  but  by  segregation  a  greenish-black  rock  may 
result,  where  hornblende  predominates,  or  a  gray  rock  where  the  feld- 
spars predominate. 
Granites. — Granites  belonging  to  several  successive  intrusions  and 
differing  often  in  habit  and  appearance  are  found.  All,  howevTer,  are 
mineralogically  very  similar  and  consist  principally  of  quartz,  feld- 
spar (mainly  orthoclase),  and  biotite.  Minor  constituents  arc  musco- 
vitc,  magnetite,  pyrite,  etc.  The  granites  differ  chiefly  in  structure 
and  in  evidences  of  shearing. 
Small  areas  of  a  muscovite-granite  are  found  in  some  minor  areas  in 
the  Georgetown  district — for  example,  in  the  northwest  corner  of  the 
area  shown  on  the  Silver  Plume  special  ma]).  This  muscovite-granite 
is  equivalent  to  the  oldest  pegmatite  and  grades  directly  into  it. 
The  biotite -granite,  which  is  the  predominant  granite  of  the  region 
described,  assumes  a  variety  of  phases — massive,  even-grained  forms, 
highly  porphyritic  phases,  or  massive  varieties  with  a  porphyritic 
habit  of  feldspars.  The  most  common  of  all  these  forms  is  a  massive 
granite  containing  phenocrysts  of  feldspar  from  three-sixteenths  to 
one-half  inch  in  length.  These  feldspars  usually  show  twinning 
after  the  Carlsbad  law.  In  some  areas  the  feldspar  phenocrysts  are 
[exceptionally  well  developed.  For  instance,  near  Empire  Pass  and 
the  town  of  Empire  phenocrysts  of  feldspar  from  one-half  inch  to  3 
inches  in  length  are  scattered  fairly  abundantly  through  a  ground- 
mass  of  biotite-granite. 
Some  forms  of  the  granite  become  so  highly  porphyritic  that  the 
bulk  of  the  rock  is  composed  of  phenocrysts  of  feldspar  in  a  finer 
groundmass.  When  this  coarse,  porphyritic  granite  has  the  some- 
what altered  condition  characteristic  of  the  vicinity  of  veins,  it  is 
termed  "  cornrock  "  by  the  miners.  Some  phases  of  the  line-grained 
porphyritic  granite,  especially  in  the  vicinity  of  Empire,  resemble 
certain  of  the  "  porphyry  "  so  completely  that  the  two  are  not  readily 
distinguished  macroscopically  when  they  occur  together.  The  porphy- 
ritic granite  also  has  a  pegmatite  phase,  which  is  segregated  from  the 
granite  in  all  degrees  of  perfection  and  in  all  textures. 
Pyrite  appears  as  an  original  accessory  of  the  granites  in  small 
quantities  throughout  the  district;    it  is  subordinate  in  amount  to  the 
^original    magnetite,    but    contemporaneous    with    it.     »Near    Empire 
pyrite  can  be  detected  in  nearly  every  specimen  of  granite.     Whether 
