386  PKE-CAMBEIAN   GEOLOGY   OF    NORTH   AMERICA. 
series  and  the  granite-gneiss  complex,  the  relations  being  those  of  pro- 
found unconformity. 
In  the  Gogebic  district  the  unconformity  between  the  Huronian 
and  the  basement  complex  was  first  thought  to  be  at  the  base  of  the 
Animikie,  but  the  cherty  limestone  was  found  by  Irving  and  Van  Hise 
to  be  below  this  break  and  to  have  such  distribution  as  to  indicate  its 
unconformity  upon  the  true  basement  complex.  Since  that  time 
Seaman  and  Sutton  have  discovered  a  quartzite  and  conglomerate 
beneath  the  cherty  limestone  of  the  lower  Huronian,  in  direct  and 
discordant  contact  on  the  schists  of  the  basement  complex,  affording 
final  proof  of  an  unconformity  which  had  been  believed  to  be  present 
from  the  distribution. 
As  before  stated,  Brooks,  Rominger,  and  Irving — the  three  who 
had  done  the  most  detailed  and  continuous  work  on  the  south  shore — 
had  at  the  outset  different  opinions  as  to  the  relations  of  the  elastics 
to  a  basement  granite  gneiss,  but  they  all  came  to  the  same  final  con- 
clusion, that  between  the  two  is  a  great  structural  break.  Pumpelly 
and  Chamberlin,  who  have  also  done  much  general  work  in  this 
region,  agree  wTith  this  conclusion.  While  this  is  true,  all  these  writ- 
ers do  not  agree  as  to  the  position  at  which  this  plane  is  found  in 
every  district. 
The  fetter  detailed  work  of  Van  Hise,  Bayley,  Clements,  Smyth, 
Merriam,  and  others  for  the  United  States  Geological  Survey  in  each 
of  these  districts  has  developed  evidence  at  many  points  of  the  exist- 
ence of  an  unconformity  at  the  base  of  the  Huronian  series. 
Coleman  and  Willmott  have  recognized  the  existence,  on  the  north- 
east shore  of  Lake  Superior,  of  a  break  between  the  Huronian  sedi- 
ments and  the  Keewatin  green  schists,  although  they  classed  the 
former  as  upper  Huronian  and  the  latter  as  lower  Huronian.  They 
do  not  recognize  the  break  between  the  Huronian  sediments  and  the 
Laurentian,  holding  that  the  latter  are  all  intrusive. 
NOTES. 
1  Third  annual  report  of  the  Geological  Survey  of  Michigan,  by  Douglass 
Houghton.     State  of  Michigan,  House  of  Representatives,  No.  S,  pp.  1-33. 
2  Fourth  annual  report  of  the  State  geologist,  Douglass  Houghton.  Idem, 
No.  27,  pp.  184.  See  also  Metalliferous  veins  of  the  Northern  Peninsula  of 
Michigan,  by  Douglass  Houghton.  Am.  Jour.  Sci.,  1st  ser.,  vol.  41,  1841,  pp. 
183-186. 
3  Geology  of  Porters  Island  and  Copper  Harbor,  by  John  Locke.  Trans.  Am. 
Phil.  Soc,  vol.  9,  1844,  pp.  311-312,  with  maps. 
4  Mineralogy  and  geology  of  Lake  Superior,  by  H.  D.  Rogers.  Proc.  Boston 
Soc.  Nat.  Hist.,  vol.  2,  1846,  pp.  124-125. 
5  Report  of  observations  made  in  the  survey  of  the  Upper  Peninsula  of  Michi- 
gan, by  John  Locke.  Senate  Docs.,  1st  sess.  30th  Cong.,  1847,  vol.  2,  No.  2,  pp. 
183-199. 
6  Report  of  J.  D.  Whitney.     Idem,  pp.  221-230. 
