32  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1905. 
loosely  consolidated  conglomerate  already  described  under  "Red  Beds."  The  Archean 
material,  of  which  this  conglomerate  seems  to  be  wholly  composed,  is  commonly  the  original 
seat  of  gold-quartz  veins.  The  wearing  down  of  such  granitic  material  is  a  common  means 
by  which  such  veins  are  worked  over  and  concentrated  in  nature.  The  conglomerate  beds 
may  be  the  base  of  the  Hahns  Peak  laccolite.  Gold  once  transported  with  these  pebbles 
from  an  older  mountain  mass  may  have  been  later  resorted  and  concentrated  on  the  slopes 
of  Hahns  Peak. 
Reports  of  assays  of  the  Hahns  Peak  Porphyry  claim  a  very  uniform  content  in  gold, 
of  from  30  to  50  cents  per  ton  throughout  this  rock.  This  indicates  the  possibility  that 
the  porphyry  may  be  the  source  of  the  metal  in  the  placers.  A  number  of  tunnels  have 
been  driven  into  the  peak  itself  in  search  of  .  he  original  gold  veins.  Most  of  this  work  has 
now  been  abandoned.  As  the  gold  in  the  porphyry  is  in  such  a  finely  divided  state  and  as 
no  nuggets  or  gold-quartz  i  earns  have  ever  been  discovered  there,  the  weight  of  this  nega- 
tive evidence  lies  against  the  theory  that  the  gold  was  derived  from  the  porphyry.  On 
the  other  hand,  the  most  productive  of  the  placer  grounds  lie  in  such  position  that  their 
material  has  been  derived  from  the  slopes  of  the  porphyry  peaks.  Seventy-five  to  ninety 
per  cent  of  the  pebbles  in  the  gravels  are  of  the  white  porphyry  itself.  It  is  probable, 
therefore,  that  the  gold  has  been  in  considerable  part  derived  from  the  Hahns  Peak  por- 
phyry. Although  the  occurrence  of  the  coarser  gold  has  not  been  discovered  within  the 
mass  of  the  porphyry  itself,  it  may  have  been  deposited  at  the  time  of  eruptive  activity 
in  and  about  what  was  then  the  contact  of  these  igneous  and  sedimentary  rocks.  Later 
erosion  and  the  present  mantle  of  slide  rock  may  have  obscured  this  zone. 
VEIN    DEPOSITS. 
In  this  field  two  lode  properties  have  shipped  ore  to  the  smelter — namely,  the  Tom 
Thumb  and  the  Minnie  D.  Both  have  been  idle  for  a  number  of  years.  The  Tom  Thumb 
was  not  visited,  and  inspection  of  the  Minnie  D.  was  limited  to  an  examination  of  the  dump. 
The  Tom  Thumb  is  situated  on  the  west  side  of  Hahns  Peak,  near  the  summit.  The  ore 
as  shipped  from  the  Tom  Thumb  is  a  galena,  much  of  which  is  altered  to  a  sand  carbonate. 
As  reported,  it  occurs  along  a  zone  of  brecciation  within  the  Hahns  Peak  porphyry.  Seen 
from  below,  the  mine  has  the  aspect  of  a  tunnel  driven  in  on  the  shearing  zone  of  a  huge 
landslip.  This  has  the  corroborative  evidence  that  the  tunnel  is  reported  to  run  in  nearly 
parallel  to  the  mountain  face.  Assays  from  a  representative  shipment  of  about  18,000 
pounds  gave  52  ounces  of  silver,  2  ounces  of  gold,  and  51.8  per  cent  of  lead  per  ton.  The 
mine  is  reported  to  have  shipped  200  tons  of  ore,  in  part  to  Denver  by  wagon  and  in  part 
to  Leadville  The  Minnie  D.  is  situated  near  Columbine,  in  an  elevated  flat  of  approxi- 
mately horizontal  rocks.  It  was  opened  by  a  vertical  shaft  325  feet  deep.  The  record  of 
strata  passed  through,  as  furnished  by  Mr.  Daniel  Stukcy,  of  Steamboat  Springs,  Colo., 
is  as  follows: 
Section  at  Minnie  D.  mine,  near  Columbine,  Colo. 
Foot. 
Black  shale 25 
Quartzite 42 
Shale 80 
Quartzite  and  conglomerate 35 
Limestone  and  dolomite 140+ 
All  these  rocks  were  found  in  the  dump.  It  is  probable  that  they  represent  the  Benton 
and  Dakota  of  the  Cretaceous  and  the  Jurassic  limestone.  The  ore  was  not  seen,  but 
$1,500  worth  is  reported  to  have  been  shipped  to  Leadville,  smelter  returns  giving  0.42 
ounce  gold  and  8.32  ounces  silver  per  ton.  At  the  time  of  visit  the  works  had  been  aban- 
doned for  several  years. 
The  Southern  Cross  tunnel  lies  on  the  south  side  of  the  peak,  about  1,000  feet  below  the 
summit. 
