312  PRE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
Irving,322  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  depo- 
sition upon  them  of  the  overlying  detritals.  In  the  Original  Huro- 
nian area  there  is  not  only  this  distinction  in  its  most  marked  form, 
but  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  phe- 
nomena are  also  seen  on  the  Canadian  Pacific  Railway  between 
Algoma  Mills  and  Sudbury.  It  is  concluded  that  the  Huronian  has 
a  group  value,  because  it  is  essentially  noncrystalline,  because  it  is 
truly  clastic  and  sedimentary,  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  Mississippi  River,  including  the  basin  of 
Lake  Superior,  is  one  geological  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 — 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  intervening  structural  break  evident.  The  peculiar 
granitoid  quartzites  which  Rominger  regards  as  having  been  pro- 
duced by  the  metamorphosed  action  of  granite  are  plainly  detrital 
derivatives  from  the  granite,  and  often  run  into  coarse  bowlder  con- 
glomerates, particular  occurrences  of  which  are  described.  Here, 
as  north  of  Lake  Huron,  proofs  of  distinctness  of  the  newer  series  are 
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  conglomerates  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  of  detrital  rocks  are  quartzites,  but 
there  are  also  present  clay  slates,  shales,  mica  schists,  and  various 
calcareous  and  dolomitic  rocks,  with  jasper  and  ferruginous  schists 
and  iron  ores  which  are  regarded  as  chemical  sediments. 
