van  hisk.]       .  DISCUSSIONS    OF    PRINCIPLES.  499 
would  thus  l>e  properly  designated  as  Lower  Hiiro nian  and  the  second 
as  Upper  Huronian.  Although  cut  locally  by  later  granites,  the  lowest 
member  of  the  inferior  series  is  separated  from  the  basement  complex 
by  a  great  unconformity.  The  upper  members  rest  uiiconformably 
below  the  Potsdam  sandstone.  The  upper  series  is  so  gently  folded 
that  the  careful  work  of  Logan  and  Murray  enabled  them  to  map  these 
rocks  in  detail  and  to  work  out  their  structure.  This  has  been  done 
for  a  considerable  district  with  as  much  certainty  and  accuracy  as  in 
many  areas  among  the  fossiliferous  xocks.  The  rocks  were  divided  into 
a  number  of  formations,  which  were  found  to  be  persistent  throughout 
the  district.  They  were  traced  as  a  broad  belt  for  several  hundred 
miles  in  a  general  direction  northeast.  Along  the  Canadian  Pacific 
railway  as  far  as  Sudbury  and  in  the  vicinity  of  Sudbury  more  than  a 
general  study  of  this  great  area  has  been  made.  The  clastic  rocks  of 
this  part  of  the  region  have  the  same  general  character  as  the  type 
district,  but  in  the  Sudbury  district  there  are  peculiar  contemporaneous 
volcanic  elastics.  In  the  little  studied  remainder  of  the  region,  as 
mapped,  numerous  granitic  and  gneissic  areas  are  included,  some  of 
which  may  be  subsequent  intrusives,  but  many  of  which  probably  rep- 
resent the  underlying  Archean. 
LAKE   SUPERIOR  REGION. 
In  the  lake  Superior  region,  between  the  Archean  and  the  Pots- 
riam  sandstone,  the  great  Algonkian  system  is  subdivided  into  three 
eries,  which  are  separated  by  very  considerable  unconformities.  The 
lowest  series  is  closely  folded,  semicrystalline,  and  consists  of  lime- 
tones,  quartzites,  mica-slates,  mica-schists,  schist-conglomerates,  and 
ferruginous  and  jaspery  beds,  intersected  by  basic  dikes,  and  in  certain 
areas  also  by  acid  eruptives.  It  includes  volcanic  elastics,  often  agglom- 
ratic,  and  a  green  chloritic,  finely  laminated  schist.  The  thickness  of 
his  series  has  not  been  worked  out  with  accuracy,  but  at  its  maxi- 
num  it  is  probably  more  than  5,000  feet.  As  the  term  Huronian  has 
jeen  for  many  years  applied  not  only  to  the  Upper  Huronian  series,  but 
:o  this  inferior  scries  about  lake  Superior,  it  is  called  Lower  Huronian. 
Above  this  series  is  a  more  gently  folded  one  of  conglomerates,  quartz- 
tes,  shales,  slates,  mica-schists,  ferruginous  beds,  interbedded  and  cut 
)y  greenstones,  the  whole  having  a  maximum  thickness  of  at  least 
2,000  feet.  In  the  Animikie  district  a  fossil  track  has  been  found,  aud 
n  the  Minnesota  quartzites  lingula-like  forms  as  well  as  an  obscure  trilo- 
)itic-looking  impression.  Carbonaceous  shales  are  abundant.  In  its  vol- 
ime,  degree  of  folding,  and  little  altered  character  the  Upper  Huronian 
sin  all  respects  like  the  upper  series  of  the  original  Huronian,  and 
:an  be  correlated  with  it  with  a  considerable  degree  of  certainty.  Above 
he  Upper  Huronian  is  the  great  Keweenawan  series,  estimated  at  its 
aaximum  to  be  50,000  feet  thick,  although  its  average  thickness  is 
auch  less.    Its  lower  division  consists  largely  of  basic  and  acid  vol- 
