806  PRE-CAMBRIAN   GEOLOGY   OF   NORTH   AMERICA. 
Range  rests  upon  the  granite  and  gneiss  containing  these  quartzite 
inclusions.  In  the  light  of  present  knowledge  concerning  pre- 
Cambrian  sediments  in  the  West  it  is  deemed  better  to  refer  the 
included  sedimentary  rocks  to  the  Algonkian.  No  Cambrian  forma- 
tion is  represented  on  the  map,  although  it  is  probable  that  a  small 
thickness  of  quartzite  and  of  cherty  limestone  below  the  Manitou 
Silurian  limestone  belongs  to  that  period. 
Cross,69  in  1895,  describes  the  geology  of  the  Cripple  Creek  district 
of  Colorado.  The  account  of  the  general  geology  is  substantially  the 
same  as  that  previously  given  by  Cross  for  the  Pikes  Peak  quadrangle, 
of  which  the  Cripple  Creek  district  is  a  part.  Granites  and  gneisses 
occupy  a  large  area  in  the  district.  Included  in  these  granites  and 
gneisses  are  large  and  small  fragments  of  quartzite,  quartz  fibrolite 
schist,  quartz  mica  schist,  and  other  similar  rocks.  It  is  believed 
that  the  quartzite  fragments  belong  to  a  great  series  of  pre-Cambrian 
(Algonkian)  sediments.  Hence  the  granites  including  such  frag- 
ments are  not  Archean;  but  they  are  older  than  the  only  Cambrian 
rocks  as  yet  identified  in  Colorado,  and  they  are  therefore  mapped 
as  Algonkian.  The  schists  are  probably  also  sedimentary,  but  it  is 
quite  possible  that  some,  if  not  all,  have  been  produced  from  Archean 
gneisses  forming  the  foundation  upon  which  the  Algonkian  sediments 
were  laid  down. 
Emmons,  Cross,  and  Eldridge,70  in  1896,  describe  and  map  the  geol- 
ogy of  the  Denver  Basin  in  Colorado.  Pre-Cambrian  rocks  form  the 
mass  of  the  Colorado  or  Front  Range  along  the  western  border  of 
the  Denver  Basin,  later  formations  resting  against  the  flanks  of  the 
mountains.  In  the  lower  canyons  of  South  Boulder  and  Coal  creeks 
are  beds  of  highly  altered  quartzite  and  conglomerate,  associated  with 
schists,  aggregating  a  thickness  of  1,000  feet,  which  occupy  a  position 
between  Triassic  sandstones  and  the  gneisses  of  the  interior  of  the 
range.  These  are  undoubtedly  sedimentary  and  are  probably  of 
Algonkian  age.  In  passing  from  these  sedimentaries  westward 
toward  the  center  of  the  range  there  appear  successively  gneisses, 
granite  gneisses,  and  massive  granite.  As  the  areas  occupied  by  the 
granites  and  sedimentaries  have  not  been  definitely  delimited,  and  as 
the  sedimentaries  occupy  but  a  small  proportion  of  the  pre-Cambrian 
area,  the  sedimentaries  are  not  mapped  as  Algonkian,  but,  with  the 
granites,  are  mapped  as  pre-Cambrian. 
Crosby,71  in  1899,  describes  the  Archean-Cambrian  contact  near 
Manitou,  Colo.  A  sandstone  of  Cambrian  age  rests  upon  an  Archean 
granite  complex.  The  granite  floor  has  very  small  erosion  inequali- 
ties. These  inequalities  are  hummocks,  not  hollows;  erosion  rem- 
nants, and  not  channels;  clearly  marking  the  end  and  not  the  begin- 
ning of  a  process  of  base-leveling. 
