308  PRE-CAMBRIAN   GEOLOGY   OF   NORTH   AMERICA. 
is  mainly  quartzose.  (4)  At  numerous  points  the  Potsdam  is  found 
to  rest  against  or  upon  the  upturned  edges  of  the  Keweenawan  series 
unconformably.  Strong  has  found  fifty-five  places  on  the  St.  Croix 
where  .the  unconformable  contact  occurs.  In  Douglas  County  are 
four  sections  in  which  the  Potsdam  sandstones  become  conglomeratic, 
bearing  material  from  the  copper-bearing  series,  but  here  the  con- 
tacts are  complicated  by  subsequent  disturbances.  There  are  several 
other  districts,  such  as  upper  St.  Croix  River,  Snake  and  Kettle 
rivers  in  Minnesota,  and  the  vicinity  of  Lake  Agogebic  in  Michigan, 
where  the  quartzose  Potsdam  sandstone  is  in  a  horizontal  position 
and  near  the  upturned  igneous  and  detrital  rocks  of  the  Keweenaw 
series.     (5)  The  foregoing  facts  are  all  consistent  with  one  another. 
(6)  The  view  is  dynamically  simple,  whereas  any  other  explanation 
implies  an  extraordinary  amount  of  local  faulting  and  disturbance. 
(7)  In  the  Grand  Canyon  of  the  Colorado  is  a  series  of  rocks  remark- 
ably similar  to  the  Keweenawan,  which  lie  directly  and  unconform- 
ably below  the  Cambrian. 
Irving  and  Van  Hise,318  in  1884,  describe  quartzites  of  many  local- 
ities belonging  to  the  rock  series  referred  to  the  Huronian  in  the  North- 
west, and  find  that  their  supposed  metamorphism  is  due  to  the  depo- 
sition of  interstitial  silica,  which  has  for  the  most  part  coordinated 
itself  with  the  original  grains,  the  forms  of  the  latter  being  as  per- 
fect as  at  the  time  of  deposition.  The  list  of  rocks  given  includes 
those  from  the  Original  Huronian,  from  the  various  iron-bearing 
regions  of  Michigan  and  Wisconsin,  from  the  Baraboo  and  Chippewa 
quartzites,  from  the  Minnesota,  Iowa,  and  Dakota  quartzites,  and 
from  other  localities. 
Irving,319  in  1885,  discusses  the  divisibility  of  the  Archean  in  the 
Northwest.  The  relations  of  the  Penokee- Gogebic  series  to  the  over- 
lying Keweenawan  and  to  the  underlying  complex  are  first  discussed. 
The  area  south  of  the  Penokee-Gogebic  series  is  found  to  consist  of 
crystalline  hornblendic,  chloride,  and  micaceous  schists,  which  locally 
show  unmistakable  evidence  of  fragmental  origin,  but  which  as  a 
whole  are  intensely  metamorphosed.  The  granites,  however,  are  con- 
sidered as  of  eruptive  origin,  as  they  intersect  intricately  the  associ- 
ated schists  at  their  contacts  with  them,  but  the  granite  is  never  found 
to  cut  the  overlying  slates.  Above  this  granite  gneiss  schist  area  is, 
first,  a  belt  of  slate  500  feet  thick,  over  this  a  belt  of  iron-bearing 
rocks  of  various  kinds,  and  above  this  quartzites  and  slates,  all  hav- 
ing a  dip  to  the  north  and  extending  for  many  miles  east  and  west. 
None  of  these  rocks  are  met  amorphic. 
North  of  this  succession  of  layers,  the  Penokee  series,  is  the  Kewee- 
nawan series,  which  appears  at  first  to  be  conformable,  but  a  closer  in- 
spection shows  that  it  is  now  in  contact  with  one  member  of  the  under- 
lying series  and  now  with  another,  even  lying  against  the  lowest 
