498  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull. 86. 
Iii  the  Hastings  series  there  are  found  considerable  areas  of  peculiar 
volcanic  elastics.  Below  these  rocks  is  a  great  complex  in  every  respect 
like  the  Laurentian  Archean  as  above  defined.  This  latter  system, 
called  usually  Lower  Laurentian,  occupies  the  main  area  of  the  region, 
and  the  elastics  are  in  a  series  of  troughs  within  it.  What  the  relations 
tire  between  the  elastics  and  the  fundamental  complex  has  not  been 
definitely  made  out,  although  Vennor  believes  that  between  the  two  is 
an  unconformity,  and  that  the  elastics  are  infolded  patches.  The  evi- 
dence given  for  this  is,  however,  rather  meager,  and  doubtless  almost 
as  good  a  case  could  be  made  out  with  present  facts  for  the  theory  of 
an  irruptive  contact  between  the  crystallines  and  the  elastics.  The 
Labrador itic  rocks  (gabbros)  found  in  this  region  need  not  be  consid- 
ered in  the  stratigraphical  succession,  as  they  are  eruptives  of  later  age 
than  the  clastic  series.  Besides  this  erur>tive,  other  acid  and  basic 
eruptives  cut  the  bedded  succession. 
To  the  elastics,  Logan,  Murray,  Vennor,  and  all  who  have  worked  in 
this  region  recognized  that  ordinary  stratigraphical  methods  could  be 
applied.  The  persistence  of  the  bands  of  limestones  is  such  as  to  ena- 
ble them  to  be  traced  for  long  distances.  Although  the  problem 
was  a  difficult  one,  a  detailed  mapping,  with  sections,  has  been  submit- 
ted for  a  small  part  of  the  area.  The  structural  relations  and  correla- 
tions which  Vennor  first  gave  differ  greatly  from  his  final  ones,  and  it 
may  be  that  even  in  the  areas  in  which  detailed  mapping  was  attempted 
that  serious  mistakes  had  been  made;  but  if  this  be  true,  the  region  is 
in  no  respect  different  from  any  other  in  which  the  structure  is  difficult. 
All  of  the  pre-Cambrian  rocks  here  found  were  supposed  by  the 
Canadian  geologists  to  be  lower  in  the  geological  column  than  the 
Huronian  of  lake  Huron.  Upon  the  last  point  no  positive  evidence  is 
at  hand.  The  two  rock  series  do  not  come  together.  In  the  most  west- 
ern Hastings  district  of  the  Laurentian,  the  elastics,  in  lithological 
character,  degree  of  crystallization,  and  amount  of  folding  are  inter- 
mediate between  the  Laurentian  and  Huronian  of  the  type  areas.  At 
first  the  Hastings  elastics  were  correlated  by  Vennor  with  the  Huron- 
ian, and  with  this  correlation  certain  of  the  official  Canadian  geologists 
now  agree,  but  afterwards  they  were  traced  with  breaks  of  not  very 
great  distances  to  the  original  Laurentian  area  and  have  always  been 
thus  mapped. 
THE   ORIGINAL  HURONIAN. 
The  Original  Huronian  of  the  north  channel  of  lake  Huron  consists : 
of  comparatively  little- altered  quartzites,  slates,  slate- conglomerates, 
graywackes,  cherts,  and  limestones  having  a  total  thickness  of  18,000 
feet,  counting  considerable  masses  of  interstratified  greenstone  which 
are  recognized  as  eruptives.  Eecent  observations  render  it  probable 
that  these  rocks  are  to  be  divided  into  two  unconformable  series,  the 
lower  of  which  is  5,000  feet  and  the  upper  13;000  feet  thick.    The  firsjb  ] 
