collier.]       TIN    DEPOSITS    OF    THE    YORK    REGION,    ALASKA.  157 
limestones  are  of  either  Devonian  or  Carboniferous  age.  The  strati- 
graphic  relation  of  this  rock  to  the  slates  and  .limestones  already 
described  has  not  been  determined. 
Two  distinct  types  of  igneous  rock  are  represented  in  the  region. 
The  first  group  comprises  basic  rocks  in  the  form  of  dikes  and  prob- 
ably sills,  more  or  less  altered,  and  sometimes  sheared,  which  may  be 
grouped  together  under  the  name  of  greenstones.  The  greenstones 
and  greenstone-schists  include  a  number  of  intrusive  masses  and  find 
their  greatest  development  in  the  slates  near  the  contact  with  the  lime- 
stones which  form  the  York  Mountains.  These  rocks  are  often  called 
granite  by  the  miners,  but  they  can  be  readily  distinguished  from  true 
granite  by  the  absence  of  quartz  and  the  generally  greenish  color. 
This  distinction  is  of  importance,  since,  so  far  as  known,  no  tin 
deposits  have  been  found  in  association  with  the  greenstone. 
The  second  group  comprises  the  acid  igneous  rocks  and  includes  a 
number  of  large  masses  of  granite,  as  well  as  dikes  of  fine-grained 
porphyritic  rock  containing  quartz  phenocrysts. 
These  rocks  find  their  greatest  development  in  Cape  Mountain, 
which  is  essentially  a  granite  stock  intruded  in  the  limestones.  The 
rock,  a  white,  coarsely  crystalline,  somewhat  porphyritic  granite,  is 
made  up  essentially  of  quartz,  microcline,  and  biotite,  but  also  con- 
tains as  accessory  minerals  albite,  muscovite,  zircon,  apatite,  tourma- 
line, pyrite,  and  fluorite.  Granitic  rocks  also  occur  in  this  district  at 
Brooks  Mountain  and  on  Lost  River,  and  similar  rocks  are  found  at 
Ears  Mountain,  Hot  Springs,  and  Asses  Ears,  northeast  of  the  York 
region,  and  in  the  Diomede  Islands,  west  of  Cape  Prince  of  Wales. 
These  granites  are  generally  unaltered  by  d}mamic  influences  such  as 
would  produce  gneissoid  and  schistose  phases,  but  in  some  places  they 
have  been  considerably  affected  by  processes  which  have  produced 
various  forms  of  granite  called  tCgreisen."  The  distribution  of  the 
granite  is  of  the  greatest  economic  importance,  since  it  is  in  granite 
dikes  and  near  their  contacts  that  the  lode  tin  deposits  have  most 
frequently  been  found.  The  prospectors  of  the  region  have  not  been 
backward  in  taking  cognizance  of  this,  and  have  made  careful  search 
for  tin  along  these  contacts. 
The  unconsolidated  gravels  and  silts  form  a  group  of  younger  sedi- 
ments. This  formation  covers  the  broad  coastal  plain  along  the  Arctic 
coast  and,  extending  southward  in  the  river  valleys,  connects  with  the 
alluvium  of  the  smaller  streams.  In  the  southern  part  of  the  York 
region  these  surficial  deposits  are  confined  to  the  creek  beds  and  nar- 
row strips  along  the  coast.  They  are  of  economic  interest  because 
in  them  is  found  the  stream  tin. 
ECONOMIC  GEOLOGY. 
The  occurrence  of  tin-bearing  lodes  in  the  bed  rock  has  been  veri- 
fied by  the  Geological  Survey  at  points  known  as  Lost  River  and  Cape 
