140  PRE-CAMBRIAN    ROCKS    OF    NORTH   AMERICA.  [bull.  86. 
lake  Superior  is  always  horizontal,  or  nearly  so,  while  the  Keweenawan 
series  at  many  points  immediately  adjacent  have  suffered  extensive  dis- 
turbance. (2)  The  difference  in  thickness  is  enormous;  the  Potsdam 
rarely  reaches  1,000  feet  thick,  while  the  Keweenawan  series  has,  in 
addition  to  a  vast  thickness  of  inter  stratified  eruptives  and  detrital 
rocks,  an  upper  portion  free  from  igneous  matter  15,000  feet  in  thick- 
ness. (3)  The  sandstones  of  the  Keweenawan  series  are  largely  com- 
posed of  silicates,  while  the  Potsdam  sandstone  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  Pots- 
dam sandstones  become  conglomeratic,  bearing  material  from  the  Cop- 
per-bearing series,  but  here  the  contacts  are  complicated  by  subsequent 
disturbances.  There  are  several  other  districts,  such  as  the  upper  St. 
Croix  river,  the  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  lying  near  the  upturned  igneous  and 
detrital  rocks  of  the  Keweenaw  series.  (5)  The  foregoing  facts  are  all! 
con sistent  with  each  other.  (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  remarkably  similar  to  the  Keweenawan,  which  lie  directly  andi 
unconformably  below  the  Cambrian. 
Irving  and  Tan  Hise,193  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  deposi- 
tion of  interstitial  silica,  which  has  for  the  most  part  coordinated  itself 
with  the  original  grains,  the  forms  of  the  latter  being  as  perfect  as  at 
the  time  of  deposition.  The  list  of  rocks  given  include  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  and  Chamberlin,194  in  1885,  give  a  systematic  account  of 
the  junction  between  the  Eastern  sandstone  and  Keweenaw  series  on 
Keweenaw  point.  Detailed  descriptions  are  given  of  the  relations  of 
the  two  series  at  Bete  Grise  bay,  at  the  Wall  ravine,  at  the  St.  Louis 
ravine,  at  the  Douglass  Houghton  ravine,  at  Torch  lake  quarry,  at  Hun- 
garian ravine,  and  at  other  points.  The  conclusions,  and  the  grounds 
upon  which  they  are  based,  of  Jackson ,  Foster  and  Whitney,  Agassiz 
Borainger,  Credner,  and  those  who  have  followed  them,  are  discussed 
in  detail. 
At  Bete  Grise  bay  the  horizontal  sandstone  is  found  upon  approach- 
ing the  melaphyre  to  become  tilted  upward,  and  along  the  junction  is 
found  the  evidence  of  faulting,  both  in  the  fluccan  of  the  sandstone  and 
in  the  broken  character  of  the  melaphyre  at  the  contact.    At  the 
