880  PRE-CAMBRTAN    GEOLOGY   OF    NORTH   AMERICA. 
sericite  schist,  phyllite,  quartzite,  and  quartz  schist,  with  many  bands 
of  yellowish-weathering  siliceous  marbles — cut  by  thick  sills  and  dikes 
of  dioritic  rock,  metamorphosed  into  an  amphibolitic  condition,  and 
by  a  batholith  of  coarse  porphyritic  granite,  which  crosses  the  bound- 
ary from  Idaho  and  forms  the  ridge  of  Rykert  Mountain  at  the 
western  slope  of  the  Kootenai  Valley  trough.  The  western  half  of 
the  region  is  for  the  most  part  occupied  by  a  younger  conformable 
group  of  formations,  including  thick  bands  of  coarse  conglomerate, 
arkoses,  volcanic  breccias  and  flows,  quartzites,  sandstones,  and  slates, 
with  rare,  thin  intercalations  of  fine-grained  crystalline  limestone. 
From  Port  Hill,  Idaho,  eastward  to  the  eastern  edge  of  the  Tobacco 
Plains  is  a  great  group  of  conformable  quartzites  and  argillites, 
which  has  been  divided  into  four  series.  The  lowest  series,  the  Cres- 
ton  quartzite,  is  composed  of  9,500  feet  of  wonderfully  homogeneous, 
highly  indurated,  thick-platy,  gray  sandstones.  Overlying  the  Cres- 
ton  quartzite  is  the  Kitchener  quartzite,  a  second  series  of  ancient, 
hard  sandstones  and  interbedded  argillites,  carrying  a  high  propor- 
tion of  disseminated  iron  oxides.  These  rusty  rocks  are  relatively 
thin  bedded  and  bear  very  abundant  sun  cracks  and  ripple  marks  on 
horizons  ranging  from  top  to  bottom  of  the  series.  The  thickness 
of  the  Kitchener  quartzite  is  about  7,400  feet.  It  is  itself  conformably 
overlain  by  at  least  3,200  feet  of  thin-bedded,  red  and  gray  argilla- 
ceous strata,  which,  together  with  subordinate  thin  beds  of  light-gray 
quartzites,  make  up  the  formation  which  Daly  calls  the  Movie 
argillite.  The  youngest  member  of  the  four  sedimentary  divisions  is 
the  Yahk  quartzite,  composed  of  white  to  gray  indurated  sandstones, 
bedded  in  thin  to  medium  courses.  The  top  of  this  series  was  not 
seen ;  the  whole  thickness  of  conformable  strata  is  nearly  20,000  feet. 
Neither  the  bottom  of  the  Creston  quartzite  nor  the  top  of  the  Yahk 
quartzite  appearing  in  the  sections,  it  is  certain  that  this  great  thick- 
ness is  only  a  minimum. 
This  is  a  continuation  of  the  sedimentary  series  mapped  in  the 
area  immediately  west  of  the  Kootenai  at  Port  Hill.  There  the 
strata  corresponding  to  the  Creston  quartzite  are  conglomerates,  grits, 
and  coarse  sandstones,  as  well  as  fine-grained  sandstones,  and  are 
thus,  on  the  whole,  notably  coarser  than  they  were  found  to  be  any- 
where in  this  season's  belt.  The  equivalent  of  the  Kitchener  quartz- 
ite is  less  strongly  charged  with  argillaceous  beds  than  is  the 
Kitchener  quartzite  east  of  the  Kootenai. 
These  facts  point  to  the  conclusion  that  the  shore  line  whence  the 
materials  composing  the  stratified  formations  were  derived  lay  to  the 
westward  and  that  the  open  sea  and  deeper  water  lay  to  the  east- 
ward of  the  western  crossing  of  Kootenai  River  at  the  international 
boundary.    This  conclusion  was  strikingly  confirmed  on  carrying  the 
