SPURR   AND 
GARREY. 
GEORGETOWN    MINING    DISTRICT,    COLORADO.  lOi 
The  felsitic  porphyries  are  those  which  have  a  fine  cryptocrys- 
talline  or  felsitic  groundmass,  either  with  no  phenocrysts  or  with 
sparsely  scattered  phenocrysts  of  one  or  more  of  the  following 
minerals:  Feldspar,  quartz,  biotite,  hornblende,  pyroxene. 
In  the  granitic  porphyries  the  number  of  these  phenocrysts  is 
increased  to  such  an  extent  that  little  or  practically  no  groundmass  is 
ieft,  and  the  rock  has  a  noncrystalline,  somewhat  granite-like  appear- 
ance. Some  dikes  observed  are  more  granitic  in  the  centers  than  on 
their  borders. 
One  phase  of  the  felsitic  porphyry  is  porphyry-obsidian  or  glassy 
porphyry,  which  is  a  result  of  the  sudden  cooling  of  borders  or  of 
narrow  tongues  of  intrusive  porphyry  masses.  This  obsidian  was 
observed  in  the  Colorado  Central  mine,  in  the  Baltimore  tunnel,  in  a 
prospect  tunnel  belonging  to  the  Elizabeth  group  at  Empire,  and  in 
a  small  tunnel  south  of  the  Silver  Plume  station. 
The  porphyries  embrace  a  wide  range  of  colors,  which  include 
gray,  brown,  green,  pink,  white,  black,  red,  and  violet.  Some  of  the 
dikes  are  fresh,  but  in  many  of  them  the  porph}7!-}7  is  in  all  stages  of 
decomposition  and  alteration.  The  agents  involved  in  the  processes 
}f  silicification,  kaolinization,  and  serpentinization  have  been  actively 
engaged  in  different  dikes  or  in  different  portions  of  the  same  dike. 
j\s  a  result  the  porphyries  vary  in  hardness  from  the  hard,  brittle, 
comparatively  fresh  rock,  or  the  silicified  and  well-indurated  altered 
bock,  to  the  soft  "  gouge  "-like  kaolinized  or  serpentinized  forms. 
Surface  outcrops  of  dikes  are  often  weathered  to  such  an  extent  that 
he  original  nature  of  the  porphyry  is  distinguishable  with  diffi- 
culty, if  at  all.  The  granitic  forms  often  weather  to  a  coarse  sand 
omposed  of  the  more  resistant  crystalline  constituents,  or  to  a  fri- 
ble  rock  but  slightly  resembling  the  original.  The  porphyry  is  often 
rery  much  discolored.  Various  shades  of  brown  and  red  are  the  com- 
i  honest  discoloration  colors;  these  are  probably  due  to  limonite  and 
Ifematite.  More  rarely  manganese  tends  to  stain  the  rocks  dark 
irown  or  black. 
I  The   dikes   of   porphyry   often   vary   in   width   within   short   dis- 
ijpices.     Usually  they  continue  in  comparatively  straight  directions, 
ut  instances  of  sharp  angular  bends  were  noted  in  two  cases.     The 
orphyries  ocur  in  the  gneisses,  both  parallel  and  transverse  to  the 
iminae,  and  also  intersect  the  granites,  diorites,  and  pegmatites. 
M  The  porphyry  is  undoubtedly  earlier  than  the   veins.     In   many 
II  kses  transverse  mineral-bearing  veins  pass  directly  from  granite  or 
1,1  peiss  areas  into  and  through  a  porphyry  dike,  and  continue  unin- 
pruptedly  into  the  granite  or  gneiss  on  the  other  side.  Instances 
"here  dikes  appear  to  cut  off  veins,  as  for  example  in  the  Pay  Rock 
id  Queen  mines,  probably  result  from  a  displacement  of  the  vein 
I 
