146  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bill. 86. 
Irving,199  in  1887,  discusses  the  separability  of  a  Huronian  group 
from  an  underlying  series.  The  character  of  the  Original  Huronian 
area  is  again  fully  discussed.  When  two  series  of  rocks  are  in  contact, 
one  of  which  is  crystalline  in  character  and  the  other  unquestionably 
of  sedimentary  origin,  there  is  presumptive  evidence  of  a  discordance 
between  them,  as,  whatever  the  origin  of  the  crystalline  schists,  their 
present  condition  indicates  the  action  of  long-continued  and  deep-seated 
processes  of  alteration  and  profound  erosion  before  the  deposition  upon 
them  of  the  overlying  detritals.  In  the  Original  Huronian  area  there 
is  not  only  this  distinction  in  its  most  marked  form,  but  also  the  actual 
contact  between  this  series  and  the  Archean  complex  is  found  near  the 
mouth  of  Thessalon  river,  the  upper  series  having  at  its  base  a  basal 
conglomerate,  the  fragments  of  which  are  plainly  derived  from  the 
foliated  crystalline  underlying  series.  Allied  phenomena  are  also' 
seen  on  the  Canadian  Pacific  railway  between  Algoma  mills  and  Sud- 
bury. It  is  concluded  that  tin1  Huronian  has  a  group  value  because  it 
is  essentially  noncrystalline,  because  it  is  truly  clastic  and  sediment- 
ary, and  because  it  has  an  immense  volume.  There  is  reason  to  believe 
that  the  area  which  stretches  from  the  north  shore  of  lake  Huron  to 
the  Mississippi  river,  including  the  basin  of  lake  Superior,  is  one  geo- 
logical basin. 
In  the  Marquette  district  the  contradictory  conclusions  reached  by 
older  writers  are  regarded  as  due  to  the  fact  that  the  stratiform  rocks 
themselves  are  made  up  of  two  entirely  distinct  sets  5  an  older  series 
of  intensely  altered  and  crumpled  crystalline  schists,  in  the  main  of 
greenish  color,  which  are  intricately  invaded  by  the  granite,  and  a  newer, 
little  altered,  mainly  fragmental  series  whose  contacts  with  the  granites 
and  the  schists  of  the  older  basement  are  such  as  to  render  an  interven- 
ing structural  break  evident.  The  peculiar  granitoid  quartzites  which 
Eominger  regards  as  having  been  produced  by  the  metamorphosed 
action  of  granite  are  plainly  detrital  derivatives  from  the  granite,  and: 
often  run  into  coarse  bowlder  conglomerates,  particular  occurrences  of 
which  are  described.  Here,  as  north  of  lake  Huron,  as  proof  of  distinct- 
ness of  the  newer  series,  is  a  general  lithological  contrast  between  the 
two;  visible  discordances;  the  penetration  of  the  lower  strata  of  the 
lower  series  by  granite  veins  which  fail  to  penetrate  the  higher  detrital 
rocks,  but  yield  fragments  to  them ;  the  development  of  true  basal  con-1; 
glomerates  at  the  contacts  of  the  two  series;  and  the  fact  that  the 
higher  detrital  rocks  are  in  contact  with  different  members  of  the  lower? 
series.  The  most  abundant  of  the  upper  series  detrital  rocks  are 
quartzites,  but  there  are  also  present  clay-slates,  shales,  mica-schists, 
and  various  calcareous  anddolomitic  rocks,  with  jasper  and  ferruginous 
schists  and  iron  ores  which  are  regarded  as  chemical  sediments. 
In  j)assing  southward  from  Marquette  great  areas  underlain  by  the 
granites,  gneisses  and  schists  of  the  older  formation  are  passed,  but' 
before  the  Menominee  river  is  reached  at  least  four  distinct  belts  are 
