AREA   NORTH   AND   NORTHEAST   OP   LAKE   HURON.  437 
glomerates  are  bowlders  and  pebbles  derived  from  the  subjacent 
gneiss  and  that  the  lower  formation  was  consequently  converted  into 
gneiss  and  probably  greatly  disturbed  before  the  upper  series  was 
laid  down.  In  1865  Logan  further  says  that  the  Huronian  uncon- 
f  ormably  overlies  the  "  Lower  Laurentian,"  and  is  believed,  although 
not  found  in  contact  with  it,  to  be  more  recent  than  the  "  Upper 
Laurentian."  These  statements  are  emphasized  by  his  sections  pub- 
lished in  1863,  which  represent  the  Huronian  as  resting  unconf orm- 
ably upon  the  "  Laurentian." 
Developments  subsequent  to  Logan  and  Murray^s  work. — The  net 
result  of  all  subsequent  work  has  been  to  confirm  the  essential  cor- 
rectness of  Logan  and  Murray's  succession  and  structure.  In  view 
of  the  early  date  of  their  work  and  the  great  difficulties  of  travel  in 
this  district  their  results  are  marvelously  accurate.  Additional  im- 
portant discoveries  as  to  the  structure  and  relations  of  the  rocks  have 
naturally  been  made.  Principal  among  these  are  the  discovery  of 
an  unconformity  within  the  Huronian  clastic  series  and  an  uncon- 
formity between  the  clastic  series  and  a  greenstone  series  below, 
mapped  by  Logan  and  Murray  as  green  chloritic  slate  and  regarded 
by  them  as  Huronian,  and  the  determination  of  the  detailed  structure 
and  thickness  of  the  different  series.  In  the  light  of  the  most  recent 
work,  the  succession  is,  regardless  of  names  and  correlation,  as  fol- 
lows, in  descending  order : 
(White  quartzite,  chert,  and  limestone. 
'(Yellow  cherty  limestone. 
1  White  quartzite. 
Red  jasper  conglomerate, 
lied  quartzite. 
3.  Upper  slate  conglomerate. 
Unconformity. 
4.  Limestone. 
f).  Lower  slate  conglomerate. 
J  White  quartzite. 
'[Gray  quartzite. 
Unconformity. 
7.  Ellipsoidal  greenstone  and  green  schist  intruded  by  granite  gneiss. 
This  succession  shows  fewer  units  than  that  of  Logan  and  Murray, 
two  or  three  of  their  units  having  been  grouped  into  single  forma- 
tions in  the  present  table.  Logan  and  Murray's  units  were  in  these 
cases  smaller  than  the  formation,  which  is  the  smallest  unit  used 
by  the  United  States  geologists  in  mapping  the  Lake  Superior 
country. 
Unconformity  within  the  Huronian. — That  an  unconformity  ex- 
isted within  the  Huronian  series  was  first  maintained  by  Alexander 
Winchell,  in  1891;  he  concluded,  mainly  from  discordance  of  bed- 
