58  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   L906;  PART    I. 
canyon  about  500  feet  deep,  extending  southwestward  from  the 
Columbia  across  the  big-bend  plateau  is  another  remarkable  feature 
which  must  be  considered  in  any  discussion  of  this  region. 
The  Columbia  and  Sanpoil  valleys,  to  an  elevation  of  1,500  feeti 
above  the  water,  are  characterized  by  broad  terraces  of  silt  and 
gravel.  The  upper  terrace  is  in  most  places  well  marked.  South  of 
the  Columbia  it  extends  for  short  distances  up  many  of  the  small 
tributary  canyons,  which  widen  out  into  straths  at  this  elevation.  M 
very  noticeable  example  of  these  straths  is  seen  on  the  road  from 
Wilbur  to  Clark,  which  follows  a  small  canyon  that  at  the  level  of  t  his 
ten-ace  widens  out  into  an  attractive  little  valley  containing  one  or 
more  farms.  North  of  the  Columbia  the  topography  previous  to  the 
terracing  was  of  an  older  type,  and  ninny  of  the  upper  terraces  extend 
for  several  miles  away  from  the  river  in  the  valleys  of  1  ributary  si  reams 
that  How  in  deep  canyons  with  silt  and  gravel  walls.  The  lower  ter- 
races, although  nearly  as  extensive  as  the  upper,  are  more  irregulaf 
and  are  not  easily  correlated  with  one  another.  Although  they  occur 
at  various  elevations  most  of  them  are  between  20  and  300  feel  above 
the  river.  Bench  lands  of  this  type,  especially  at  the  lower  elevaJ 
t  ions,  are  in  many  places  easily  irrigated  from  small  1  ributary  si  reams] 
Such  lands  south  of  the4  Columbia  are  \\s(h\  almost  exclusively  for  fruit 
culture  and  are  valued  ai  s  lo  i,>  s  inn  an  acre. 
GEOLOGY. 
GEOLOGIC    FORMATIONS. 
Nearly  all  the  rocks  north  of  the  Columbia  and  in  the  bed  of  its 
canyon  are  schists  or  intrusives  of  various  kinds,  which  probably 
range  in  age  from  lower  Paleozoic  to  early  Tertiary.  They  have  not 
vet  been  studied  in  sufficienl  detail  to  differentiate  the  areas  of  the 
various  types.  South  of  the  Columbia  these  crystalline  rocks  arc 
buried  beneath  basalts  and  interbedde_d  sediments  of  Miocene  age, 
which  haw  been  described  as  the  Yakima  basalt."  Generally  these 
basalts  terminate  at  the  south  side  of  I  he  Columbia  gorge,  though 
there  are  a  few  small  areas  within  the  Colville  Reservation  capped  by 
them.  Easl  of  the  reservation  the  basalts  extend  a  few  miles  north  of 
Spokane  River,  where  t  hey  overlap,  t  he  crystalline  rocks  thinning  out 
at  their  northern  edge.  rrhe  sands,  silts,  and  gravels  forming  the 
benches  and  terraces  along  Columbia  and  Sanpoil  rivers  constitute 
the  youngesl  formations  in  the  region.  In  general  the  upper  terraces 
are  composed  of  line,  lighl  gray  or-  brown  silt  and  sand  containing  only 
a  small  percentage  of  gravel.  As  will  be  shown,  they  are  indirectly 
the  products  of  glaciation.  The  sediments  of  the  lower  benches  con- 
tain more  or  less  false-bedded  gravel  and  sand  and  are  river  deposits 
a  Smith,  G.  O.   Water-Sup.  and  Irr.  Paper  No    15  I     -    Geol.  Survey,  1901,  pp.  15-17.    Calkins,  F.  C., 
Water-Sup.  and  Lrr.  Paper  No.  118,  U.  S.  Geol.  Survey,  L905,  pp.  30   15 
