collier]  TIN    DEPOSITS    OF    THE    YORK    REGION,   ALASKA.  159 
of  the  limestone  and  granite.  Many  bowlders  and  pebbles  containing 
minerals  derived  from  this  contact  have  been  found  along  Tin  Creek, 
which  follows  the  contact  for  some  distance.  The  more  common  of 
these  are  tourmaline,  garnet,  epidote,  and  fluorite.  Large  bowlders 
have  been  found  which  are  wholly  composed  of  minerals  of  this  kind. 
A  white  porphyritic  dike  cutting  the  limestone  4  or  5  miles  from  the 
coast  forms  the  present  focal  point  of  interest  to  the  tin  miner.  This 
dike,  which  is  about  100  feet  wide,  has  been  traced  from  Tin  Creek  on 
the  east  to  Cassiterite  Creek  on  the  west,  a  distance  of  about  a  mile. 
Tin  ore  has  been  found  on  the  croppings  of  this  dike  and  strewn  over 
the  surface  along  its  course,  but  varies  in  general  appearance  and 
character.  Some  of  the  weathered  ore  from  the  croppings  is  highly 
siliceous  and  has  the  appearance  of  weathered  iron-stained  vein  quartz, 
with  small  crystals  of  black  cassiterite  distributed  through  it,  while 
other  specimens  show  clearly  their  granitic  origin,  but  contain  com- 
paratively little  quartz.  In  the  ore  of  the  latter  type  the  cassiterite 
occurs  both  as  disseminated  crystals,  varying  from  the  size  of  a  pin 
head  to  a  walnut,  and  as  veinlets  and  irregular  masses.  Some  of  this 
granitic  ore  that  was  prepared  and  examined  microscopically  is  found 
to  be  very  much  altered  from  its  original  character,  now  consisting 
essentially  of  calcite,  fluorite,  quartz,  and  large  crystals  of  lithia  mica; 
no  feldspars  remain,  and  the  quartz  is  probably  secondary.  In  addi- 
tion to  the  cassiterite,  tourmaline,  p}^rite,  galena,  and  garnet  occur  as 
accessory  minerals.  The  siliceous  ore  mentioned  above,  when  exam- 
ined with  the  hand  lens,  occasionally  shows  spangles  of  free  gold.  A 
small  piece,  but  not  a  commercial  sample,  was  assayed  for  gold  and 
silver,  and  gave  0.36  of  an  ounce  of  gold  per  ton  and  a  trace  of  silver. 
Assays  reported  to  the  writer  show  the  presence  of  gold,  though  in 
less  amounts  than  the  above.  The  occurrence  of  so  much  gold  asso- 
ciated with  the  cassiterite  is  unusual  in  tin  deposits  and  merits  further 
investigation.  Pannings  from  the  croppings  of  this  ledge  have  yielded 
tin  ore  in  angular  unworn  crystals.  One  specimen  of  placer  tin  of 
this  kind,  obtained  near  the  croppings  of  the  large  dike  described 
above,  consists  mainly  of  cassiterite,  but  contains  also  wolframite  and 
garnet. 
To  summarize  briefly  the  evidence  in  regard  to  this  deposit,  the  tin 
ore  is,  in  part  at  least,  essentially  an  altered  granite-porphyry,  or 
"greisen,"  having  crystals  of  cassiterite  disseminated  through  it.  This 
greisen  forms  a  dike,  which  has  been  followed  eastward  from  Cassiterite 
Creek  to  Tin  Creek,  a  distance  of  about  6,000  feet,  and  has  been 
examined  at  its  eastern  and  western  ends  by  United  States  geologists, 
who  collected  specimens  of  tin  ore  from  it  near  Cassiterite  Creek. 
At  its  eastern  end,  near  Tin  Creek,  no  specimens  containing  cassiterite 
were  collected  by  members  of  the  Geological  Survey,  but  such  speci- 
mens have  been  obtained  by  others. 
