LAKE    SUPERIOR   REGION.  161 
south  end  of  the  Republic  trough  the  variation  was  more  than  1,500 
feet. 
The  Marquette  district  has  been  folded  in  a  complex  manner.  The 
largest  but  least  conspicuous  fold  of  the  district  is  an  anticline  having 
a  north-south  axis,  running  through  the  city  of  Marquette.  This 
great  fold  has,  especially  near  its  crown — that  is,  for  the  eastern  6  or 
8  miles  of  the  district — folds  of  the  second  order  superimposed  upon 
it,  making  this  part  of  the  fold  an  anticlinorium.  The  other  major 
anticline  belonging  to  this  system  of  folds  is  one  running  north  and 
south  through  the  east  end  of  Mich iga mine  Lake.  The  major  part  of 
the  district  has  been  affected,  however,  by  much  more  effective  pres- 
sure in  a  north-south  direction,  so  that  the  folds  in  an  east-west  direc- 
tion are  much  more  conspicuous  than  the  north-south  folds  of  greater 
wave  length  and  greater  amplitude.  As  a  result  of  the  north-south 
pressure,  the  Upper  and  Lowrer  Marquette  series  together  have  been 
bent  into  a  great  abnormal  synclinorium.  This  synclinorium  is  of  a 
peculiar  and  complicated  character.  The  Algonkian  rocks  on  either 
side  of  the  trough  have  moved  over  the  more  rigid  Archean  granite, 
and,  as  a  consequence,  on  each  side  of  the  Algonkian  trough  a  series 
of  overfolds  plunge  steeply  toward  its  center,  producing  a  structure 
resembling  in  this  respect  the  composed  fan  structure  of  the  Alps. 
There  is,  however,  this  great  difference  between  the  Marquette  struc- 
ture and  that  of  the  Alps,  that  in  passing  from  the  sides  of  the  trough 
toward  the  center  newer  rocks  appear  rather  than  older  ones,  so  that 
in  the  center  of  the  synclinorium  the  youngest  rocks  are  found.  It  is 
as  if  the  composed  fan  folds  of  the  Alps  were  sagged  downward,  so 
that  the  structure  as  a  whole  is  a  synclinorium  rather  than  an  anti- 
clinorium. This  form  of  folding  has  been  elsewhere  defined  by  Van 
Hise  °  as  an  abnormal  synclinorium.  The  folding  is  closer  in  the 
western  part  of  the  district  than  to  the  east.  The  strikes  of  most  of 
the  exposures  of  the  district  are  mainly  controlled  by  the  east-west 
folds,  but,  at  the  east  and  west  ends  of  the  areas  of  the  formations 
the  larger  north-south  folds  already  described  control. 
Wadsworth,77  in  1898,  describes  the  origin  and  mode  of  occurrence 
of  the  Lake  Superior  copper  deposits  and  the  age  of  the  copper-bear- 
ing series.  A  reexamination  of  the  Douglass  Houghton  and  Hunga- 
rian River  areas  shows  that  the  Eastern  sandstone  passes  under  the 
lavas  with  increasing  dip,  and  that  the  junction  is  not  a  fault  junction 
but  that  of  a  lava  flow  upon  an  underlying  soft  sand  and  mud.  It  is 
held  that  the  Eastern  sandstone  is  of  Potsdam  age,  and  underlies  the 
copper-bearing  series,  the  first  lava  of  that  series  having  flowed  out 
upon  the  sandstone.    The  basaltic  rocks  forming  the  Bohemian  Moun- 
a  Van  IIis<\  C.  K.,  Principles  of  North  American  pro-Cambrian  geology:  Sixteenth  Ann. 
Kept.  I'.  S.  Geol.  Survey,  pt.  1,  189G,  p.  612. 
55721— Bull.  360—09 11 
