30  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1905. 
The  structure  that  this  peak  suggests  is  that  of  a  laeeolite — a  lenticular  body  of  igneous 
rock  intruded  into  the  bedding  planes  of  a  sedimentary  series.  Its  formation  is  explained 
as  the  result  of  an  ascending  stream  of  molten  rock,  which  expands  laterally  before  reach- 
ing the  surface  and,  by  lifting  the  overlying  beds,  accumulates  in  a  great  mass  in  the  chamber 
so  made.  Subsequent  erosion  of  the  overlying  sediments  may  expose  various  portions  of 
its  irregular  surface.  That  the  Hahns  Peak  mass  is  of  a  lenticular  character  is  no  more  than 
supposition  as  a  probable  explanation  of  its  relations  to  the  surrounding  sediments,  for  no 
direct  evidence  as  to  the  character  of  the  base  of  this  mass  was  observed. 
The  second  intrusive  is  a  typical  olivine  basalt."  It  is  shown  in  a  prominent  dike  north  of 
Hahns  Peak  between  Columbine  and  Little  Red  Park.  This  rock  forms  a  large  part  of  the 
Elkhead  Mountains  to  the  west  and  southwest  of  Hahns  Peak.  Both  of  these  intrusives 
cut  through  Cretaceous  rocks  and  arc  therefore  of  later  age.  By  analogy  with  similar 
occurrences  elsewhere  they  are  probably  of  Tertiary  age. 
SEDIMENTARY    ROCKS. 
Red  Beds. — The  oldest  of  the  sedimentary  scries  here  shown  are  the  "lied  Beds"  of 
probable  Triassic  age.  They  are  composed  of  red  sandstone,  red  shale,  and  conglomerates 
with  well-rounded  pebbles  of  granitic  material,  such  as  make  up  this  formation  in  many 
parts  of  the  West.  The  one  undoubted  exposure  of  these  beds  in  this  field  is  in  normal 
position,  lying  next  the  Archean  rocks  at  the  western  foot  of  Farwell  Mountain.  On  the 
road  between  Hahns  Peak  post-office  and  Columbine  are  sonic  exposures  of  variegated 
shaly  soil  and  some  prospect  pits  in  material  that  strongly  suggests  the  Jurassic  formation, 
but  these  are  not  represented  on  the  map. 
Loosely  cemented,  very  coarse  conglomerates  of  rounded  pebbles  of  granitic  rocks  are 
also  shown  in  several  places.  These  have  likewise  been  omitted  from  the  map  on  account 
of  uncertainty  as  to  their  extent  and  proper  classification.  They  were  found  high  on  the 
mountain  slopes  and  in  the  ravines  in  the  vicinity  of  the  white  porphyry.  Whether  they 
belong  with  the  "Red  Beds"  at  the  base  of  the  sedimentary  series  or  are  scattered  patches 
of  overlying  and  considerably  later  Tertiary  beds  could  not  be  determined.  If  they  do 
belong  to  the  "Red  Beds"  series,  it  seems  strange  that  they  should  have  been  so  little 
altered  by  their  proximity  to  the  intrusive  rock  of  Halms  Peak,  for  the  Dakota  quartzite 
above  the  "Red  Beds"  has  been  very  completely  vitrified. 
Dakota  formation. — The  next  formation  showing  in  outcrop  is  the  Dakota  quartzite  and 
conglomerate.  It  is  composed  of  beds  of  conglomerate,  characterized  by  perfectly  rounded 
pebbles  of  exceedingly  hard  material,  of  white  and  gray  quartz  and  a  darker  flint.  Its 
upper  portion  is  white,  even-grained  sandstone  and  quartzite.  Its  unusually  complete 
silicification  in  most  parts  of  this  field  is  attributed  to  the  effect  of  the  intrusion  of  the 
Hahns  Peak  porphyry.  It  lies  nearly  flat  over  a  considerable  area  about  Columbine, 
rising  on  the  north  and  west  flanks  of  the  peak  to  within  a  few  hundred  feet  of  the  summit. 
In  scattered  patches  it  also  shows  at  many  other  places  throughout  the  field.  The  shaft 
known  as  the  "Minnie  D,"just  north  of  Columbine,  was  put  down  through  this  formation 
where  it  is  nearly  horizontal  and  should  give  a  good  measure  of  its  thickness.  This  shaft 
passed  through  the  Dakota  into  a  compact  blue  limestone  below,  which  is  probably  the 
Jurassic  limestone,  found  elsewhere  immediately  below  the  Cretaceous.  A  record  of  the 
measurements  in  this  shaft  is  given  later  under  a  description  of  this  property. 
Benton  shale. — Many  of  the  shafts  about  Columbine  pass  through  a  considerable  thickness 
of  black  shale  before  striking  the  Dakota  quartzite.  This  shale  is  similar  in  character  and 
position  to  that  called  Benton  east  of  the  mountains.  Where  not  baked  by  igneous  eon- 
tact,  it  is  soft  and  very  dark,  with  calcareous  seams.  Shells  of  oysters  and  Inoceramus 
were  found  in  these  calcareous  seams.  The  upper  valley  of  Beaver  Creek  seems  to  have 
been  opened  out  on  a  narrow  syncline  of  these  shales,  lying  between  Farwcll  Mountain  and 
a  Thiii  sections  of  this  rock  were  also  examined  by  Mr.  Albert  Johannsen. 
