810  PRE-CAMBRIAN    GEOLOGY   OF    NORTH    AMERICA. 
but  without  the  inclusions,  as  having  been  very  likely  originally 
arkoses,  sandstones,  and  shales.  The  interbanded  relation  of  these 
different  phases  is  favorable  to  this  explanation;  and  certain  highly 
siliceous  bands  have  the  composition  of  quartzites. 
Interbedded  with  these  schist  series  is  a  peculiar  hornblende  gneiss 
characterized  by  many  facies.  Coarse,  pegmatite-like  growths  of 
quartz  and  epidote,  epidote  and  garnet,  garnet  and  quartz,  and  horn- 
blende and  plagioclase  and  lime  silicate  rocks  occur.  The  wide  varia- 
bility of  these  rocks,  often  within  short  distances,  together  with  their 
character,  indicates  that  they  are  much-metamorphosed,  impure 
calcareous  sediments. 
Dikes  and  sheets  of  a  hornblende  gneiss  cut  both  the  various  mem- 
bers of  the  Idaho  Springs  formation  and  the  older  hornblende  gneiss 
above  mentioned.  This  usually  has  a  well-developed  schistosity,  but 
is  occasionally  rather  massive.     It  is  probably  mashed  diabase. 
The  above  formations  are  cut  by  a  quartz  monzonite  gneiss  whose 
hornblende  has  been  altered  partially  or  wholly  to  biotite. 
The  next  oldest  formation  is  a  fine-grained,  gneissoid,  biotite-poor 
granite,  often  with  plagioclase  equaling  orthoclase  in  abundance. 
The  secondary  development  of  microcline  is  characteristic. 
A  coarse-grained  quartz  monzonite,  sometimes  massive,  sometimes 
sheared,  and  often  porphyritic,  cuts  the  above  formations.  This 
monzonite  is  the  most  extensive  formation  in  the  quadrangle,  forming 
a  batholith,  from  which  the  gneisses  have  quaquaversal  dips. 
A  quartz-bearing  diorite  of  approximately  equal  age  with  the  last- 
mentioned  rock  occurs  in  small  stocklike  areas,  more  especially  in 
the  northern  part  of  the  quadrangle.  This  varies  from  a  pure  horn- 
blende rock  to  a  practically  pure  plagioclase  rock.  Certainly  some 
of  the  hornblende  is  an  alteration  product  of  pyroxene. 
The  next  3^oungest  formation  is  the  pink  Rosalie  granite,  exceed- 
ingly coarse,  the  slightly  smoky  quartz  and  the  biotite  often  being 
half  an  inch  across  and  the  pink  or  pinkish-gray  feldspar  an  inch  in 
length.  This  formation  forms  several  small  batholiths  in  the  south- 
ern portion  of  the  quadrangle.  From  the  southeast  corner  of  the 
quadrangle  it  extends  eastward,  outcropping  in  characteristic  dome- 
like forms. 
Cutting  the  preceding  formations,  with  the  possible  exception  of 
the  last,  is  a  massive  porphyritic  biotite  granite,  in  some  phases  a 
granite  porphyry,  which  has  been  called  the  Silver  Plume  granite. 
The  phenocrysts,  which  are  more  or  less  perfect  in  form,  and  usually 
Carlsbad  twinned,  have,  near  the  contacts  with  the  older  rocks,  a 
marked  parallel  alignment.  The  unmashed  condition  of  both  pheno- 
crysts and  groundmass  shows  that  this  orientation  is  an  original  struc- 
ture.    This  granite  is  quarried  near  Silver  Plume. 
