126  PRE-CAMBRTAN    GEOLOGY   OF    NORTH   AMERICA. 
the  horizontal  sandstones  are  found  abutting  against  the  uplifted 
edges  of  a  different  rock  series,  the  copper-bearing  rocks.  The  ab- 
rupt edges  of  the  strata  look  to  the  southeast,  and  their  dip  is  in  the 
opposite  direction  at  angles  varying  from  70°  to  40°.  The  uncon- 
formable abutment  of  the  sandstones  against  the  trappean  series  is 
plainly  observed  at  several  places  near  Houghton,  on  the  property 
of  the  Isle  Royale  Company,  near  the  stamp  works  of  the  Calumet 
and  Hecla  mines,  on  the  railroad  coming  down  from  the  mines  to 
the  stamp  works,  and  on  the  Sheldon  and  Columbia  property. 
Rominger,37  in  1876,  describes  the  red  Lake  Superior  sandstone  as 
unconformably  abutting  against  or  overlapping  the  trap  rock  with 
horizontally  disposed  layers.  On  the  western  slope  of  the  ridge  the 
trap  rocks  are  conformably  overlain  by  sandstones,  conglomerates, 
and  slates,  the  age  of  which  is  intermediate  between  the  trap  and  the 
horizontal  sandstone,  but  between  all  three  there  are  such  great 
lithological  affinities  that  it  is  natural  to  regard  them  as  consecutive 
products  of  one  and  the  same  epochs.  The  absence  of  trappean  rocks 
distinguishes  the  upper  division  from  the  lower. 
The  Huron  Mountains  are  a  crystalline  granitic  and  dioritic  Lauren- 
tian  rock  series.  These  granitic  rocks  are  surrounded  by  a  narrow  belt 
of  the  horizontal  red  sandstone  of  Lake  Superior,  which  abuts  uncon- 
formably against  them.  The  Huronian  rock  series,  with  uplifted  beds 
alternating  with  slate  rocks,  quartzites,  diorites,  and  jaspery  strata, 
with  seams  of  iron  ore,  lean  unconformably  against  the  granitic  series. 
Brooks,38  in  1876,  places  granite  as  the  youngest  Huronian  rock 
south  of  Lake  Superior.  This  granite  occurs  as  the  uppermost  mem- 
ber of  the  Menominee  and  Penokee  series,  but  in  the  latter  it  thins  out 
and  disappears  before  reaching  the  Gogebic  region.  The  lithological 
character  of  this  granite  belt  bears  a  general  resemblance  to  the  Lau- 
rentian  rocks.  This  granite,  from  the  fact  that  it  does  not  give  off 
dikes  cutting  the  copper-bearing  series,  is  believed  to  be  earlier  than 
the  latter.  Although  there  is  approximate  conformability  in  strike 
and  dip,  there  is  probably  an  unconformity  between  the  copper-bear- 
ing rocks  and  the  Penokee-Gogebic  Huronian,  as  shown  by  the  fact 
that  the  former  series  is  in  contact  in  different  places  with  various 
members  of  the  Huronian.  There  is  also  an  unconformity  between 
the  Huronian  and  the  Laurentian. 
As  supporting  the  view  that  the  pre-Silurian  systems  are  distinct 
periods,  attention  is  called  to  the  lithological  differences  between  the 
three  series,  as  well  as  to  the  intensity  of  folding  to  which  they  have 
been  subjected.  The  detrital  members  of  the  copper-bearing  series 
consist  of  friable  sandstone  showing  no  greater  metamorphism  than 
the  Silurian,  and  it  is  folded  only  in  regular  magnificent  sweeps,  the 
same  strike  and  dip  continuing  in  some  cases  for  about  150  miles.  The 
Huronian  series  consists  of  greenstones,  various  schists,  clay  slates, 
quartzites,  marbles,  with  gneisses  and  granites  containing  no  copper, 
