152  PKE-CAM  BRIAN    GEOLOGY    OF    NORTH    AMERICA. 
Tamarack  mines.  The  strata  here  consist  of  interst  rati  tied  traps, 
amygdaloids,  sandstones,  and  conglomerates.  Deep  in  the  series 
there  is  less  amygdaloid,  and  it  is  suggested  that  the  amygdaloids  are 
largely  pseudo-amygdaloidal,  their  development  being  dependent 
upon  subsurface  weathering.  It  is  found  that  the  conglomerates 
approach  each  other  in  passing  from  the  north  toward  the  south,  due 
to  the  thinning  of  the  igneous  beds.  The  Eastern  sandstone,  some- 
what remote  from  the  line  of  junction  with  the  Keweenawan  series, 
has  at  places  a  dip  toward  the  traps  as  high  as  10°  or  12°.  At  Lake 
Linden  this  formation  is  shown  by  boring  to  be  at  least  1,500  feet 
thick,  and  to  consist  of  red  sandstone  with  several  streaks  of  marl. 
The  likeness  of  this  sandstone  to  the  Upper  Keweenawan  sandstone, 
the  faulting  along  or  near  the  contact  line  of  the  two  formations,  and 
the  thinning  of  the  traps  and  amygdaloids  in  passing  toward  the 
Eastern  sandstone,  seem  to  strongly  favor  the  theory  that  the  two 
formations  are  of  the  same  age. 
Van  Hise,g8  in  1894,  describes  the  rocks  of  the  Marquette  district 
as  constituting  a  great  synclinorium.  The  axial  planes  of  the  minor 
folds  on  the  sides  dip  toward  the  center  of  the  synclinorium,  thus  re- 
sembling the  fan  structure  of  the  Alps;  but  there  is  the  great  differ- 
ence that  the  major  fold  is  a  synclinorium  rather  than  an  anticli- 
norium.     This  kind  of  fold  may  be  called  the  Marquette  type. 
Smyth  (H.  L.),69  in  1804,  compares  the  Lower  Menominee  and 
Lower  Marquette  scries  in  Michigan.  The  Lower  Menominee  series 
consists,  in  ascending  order,  of — 
1.  A  basal  qnartzite,  rarely  conglomeratic,  having  a  thickness  of  700  to  1,000 
feet. 
2.  A  crystalline  limestone,  averaging  700  to  1,000  feet  in  thickness. 
3.  Red,  black,  and  green  slates,  not  known  to  exceed  200  or  300  feet  in  thick- 
ness, and  containing  the  iron  formation  that  gives  the  rich  ores  of  Iron  Moun- 
tain and  Norway.  Toward  the  north  the  horizon  of  the  slates  is  in  part  occu- 
pied by  later  eruptiyes  that  rapidly  increase  in  thickness  and  attain  a  maximum 
of  nearly  2,000   feet. 
4.  The  Michigamme  Mountain  jasper.  The  least  modified  phase  seems  to  be, 
in  part  at  least,  a  sediment.  The  most  highly  altered  kind  is  like  the  banded, 
specular  jasper  of  Republic. 
The  Lower  Marquette  series  in  the  western  part  of  the  Marquette 
area  consists,  in  ascending  order,  of — 
1.  A  basal  conglomerate,  qnartzite,  and  quartz  scliist,  probably  less  than 
100  feet. 
2.  An  iron-bearing  formation,  which  may  be  divided  into  a  lower  actinolitic 
schist  and  an  upper  banded  red  jasper  and  specular  hematite.  This  member 
has  a  maximum  thickness  of  more  than  1,000  feet. 
The  magnetic  jasper  of  Michigamme  Mountain  has  been  traced, 
by  means  of  outcrops  and  magnetic  work,  within  H  to  2  miles  of  the 
