44  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1905. 
IGNEOUS    ROCKS. 
The  igneous  rocks  of  the  district  fall  naturally  into  two  general  groups — "granitic  rocks; " 
and  basic  sills  and  dikes. 
Granitic  rocks. — A  broad  belt  of  intrusive  granitic  rock  extends  along  the  western  bound- 
ary of  the  Algonkian  area.  Another  large  intrusion  of  granite  is  found  just  north  of  Bull 
Lake  in  the  Cabinet  Mountains,  and  a  third  great  granitic  mass  outcrops  in  the  basins  of 
Lightning  and  Callahan  creeks.  A  vast  body  of  very  hard,  tough  porphyritic  rock,  proba- 
bly granite-diorite-porphyry,  forms  a  mountain  peak  on  the  eastern  shore  of  Lake  Pend 
Oreille. 
Basic  sills  and  dikes. — The  intrusive  sills  which  sometimes  occur  in  the  bedding  planes  of 
the  sedimentary  rocks  of  the  district  are  well  shown  at  the  mouth  of  Mooyie  River  and  on 
the  boundary  5  miles  east  of  that  river.  Basic  dikes  also  cut  the  formations  in  many  direc- 
tions and  in  some  instances  have  mineral  deposits  associated  with  them. 
Basalt. — Several  large  basalt  terraces  occur  around  the  south  end  of  Lake  Pend  Oreille. 
They  are  remnants  of  the  great  Miocene  basalt  Hoods  which  came  in  from  the  south. 
QUATERNARY    GRAVEL. 
The  Quaternary  gravels  are  mentioned  on  account  of  their  economic  interest.  They 
form  extensive  benches  high  above  present  river  levels.  Streams  coming  in  contact  with 
these  benches  have  reworked  the  material  and  gradually  concentrated  the  heavier  metals 
in  the  gravel  bars  which  formed  in  their  channels.  A  few  lean  diggings  of  placer  gold 
have  been  formed  in  this  way,  notably  on  Mooyie  River  and  Vermilion  Creek.  Placer 
mining,  however,  has  not  been  very  productive  in  the  district  as  a  whole. 
STRUCTURE. 
A  glance  at  the  accompanying  map  (fig.  2)  will  show  the  location  and  direction  of  the 
principal  faults  better  than  they  can  be  described.  The  displacement  in  some  of  these 
normal  breaks  has  been  very  great,  upper  beds  having  been  brought  into  juxtaposition 
with  beds  formerly  many  thousand  feet  below  them.  The  friction  caused  by  these  slips 
has  formed  shattered  zones  on  each  side  of  the  fissure.  Rarely  a  ridge  of  quartz-breccia 
marks  a  fault.  Here  the  fragments  have  been  cemented  by  silica-bearing  solutions  into 
a  mass  which  successfully  resists  weathering.  Oftener,  however,  the  crushed  fragments 
disintegrate  and  weather  away,  marking  the  break  with  a  V-shaped  valley.  Oxidizing 
surface  waters  percolating  freely  through  these  fragments  aid  the  prospector  to  locate 
the  fault  by  marking  it  with  an  iron  capping.  These  faults  have  a  direct  economic  impor- 
tance in  that  they  formed  channels  of  circulation  in  which  the  mineral-bearing  solutions 
were  able  to  deposit  their  loads.  Thus  were  formed  most  of  the  present  mineral  veins 
of  the  district.  It  should  be  noted,  however,  that  the  large  faults  are  not  so  apt  to  contain 
economic  deposits  as  the  smaller  ones.  This  may  be  due  to  the  great  area  of  the  large 
fault  fissures  and  of  their  brecciated  zones,  which  would  render  extremely  difficult  the 
collection  by  percolating  waters  of  enough  mineral  to  make  an  ore  deposit.  The  Snowshoe 
fault  is  no  exception  to  this,  because,  though  very  large,  it  is  clearly  cut  and  has  no  brecci- 
ated zone;  hence  the  portion  subject  to  mineralization  is  quite  restricted. 
MINERAL  RESOURCES. 
General  statement. — To  the  northwest  of  the  region  under  survey  are  the  great  mineral 
districts  of  Slocan,  Arrow  Lake,  Lardeau,  and  Revelstoke,  in  southern  British  Columbia. 
To  the  southeast  are  the  famous  Coeur  d'Alene  silver-lead  mines.  Indeed,  a  fairly  complete 
series  of  mineralized  areas  may  be  traced  between  these  two  mining  districts.  This  con- 
necting mineral  zone  may  be  observed  on  the  northeast  slope  of  the  Coeur  d'Alene  Moun- 
