40  rRE-CAMBRIAN    ROCKS-  OF    NORTH    AMERICA.  [bum,. 86. 
Laurentian  gneiss  is  followed  by  a  slate  conglomerate  which  contains 
pebbles  and  bowlders  sometimes  a  foot  in  diameter  of  the  subjacent 
gneiss.  The  Huronian  of  lake  Huron  is  correlated  with  the  lower  cop- 
per-bearing rocks  of  lake  Superior.  Several  detailed  sections  are 
described.  The  general  sections  represent  the  Huronian  series  as 
resting  unconformably  upon  the  Laurentian. 
Logan,45  in  1865,  states  that  the  horizontal  strata,  which  form  the 
base  of  the  Lower  Silurian  in  western  Canada,  rest  upon  the  upturned 
edges  of  the  Huronian  series,  which  in  its  turn  unconformably  overlies 
the  Lower  Laurentian.  The  Huronian  is  believed  to  be  more  recent 
than  the  Upper  Laurentian  series,  although  the  two  formations  have 
never  yet  been  seen  in  contact. 
Selwyn,30  in  1884,  west  of  Wahnahpitae  river,  on  the  Canadian  Pacific 
railway,  for  80  miles,  finds  Laurentian  rocks,  which  consist  of  felsites 
or  felsitic  quartzites,  thin  bedded  quartzites  which  hold  angular  frag- 
ments of  granite  and  gneiss,  diorite  and  diabase,  with  a  series  of  coarse 
and  fine  fragmental  beds  varying  in  character  from  a  fine  ash  to  coarse 
agglomerate. 
Irving,46  in  1887,  summarizes  the  information  of  the  Canadian  Survey 
with  reference  to  the  Huronian  of  lake  Huron,  and  describes  a  contact 
near  Thessalon  river  between  the  underlying  gneissic  series  and  the 
overlying  Huronian.  Here  a  basal  conglomerate,  containing  partly 
rounded  and  angular  fragments  up  to  2  feet  in  diameter,  largely  derived 
from  the  immediately  underlying  gneiss,  rests  directly  upon  the  up- 
turned edges  of  the  gneissic  series.  Such  a  contact  indicates  a  great 
structural  break,  whether  the  underlying  gneissic  series  is  of  eruptive 
or  of  sedimentary  origin.  If  sedimentary,  it  must  have  been  metamor- 
phosed to  its  present  crystalline  condition  and  upturned  before  the  frag- 
mentals  were  deposited  upon  it;  if  eruptive,  its  coarsely  crystalline 
character  shows  that  it  belongs  to  the  deep-seated  rocks  which  must 
have  crystallized  at  depth,  and  therefore  has  been  subjected  to  great 
erosion  in  order  that  this  class  of  rocks  may  be  found  at  the  surface. 
Such  a  contact  is  also  found  at  two  or  three  other  points  on  the  Algoma 
branch  of  the  Canadian  Pacific  railway,  and  particularly  at  the  vicinity 
of  the  mouth  of  Serpent  river.  Logan's2  green,  chloritic  slate  is  com- 
posed of  diabase  sheets*  and  a  little  interleaved  fragmental  material, 
perhaps  partly  volcanic  ash. 
Winchell  (N.  H.),47  in  1888,  describes  many  localities  within  the 
Original  Huronian.  Logan's  chloritic  slates,  as  well  as  the  greenstones, 
are  regarded  as  accidental  features,  the  former  being  a  part  of  the  basic 
eruptive  rocks  of  the  region.  Vast  outflows  of  greenstone  cover  many 
square  miles  in  the  Thessalon  valley  and  constitute  hill  ranges  as  con- 
spicuous as  those  of  any  hill  rock  in  the  region.  This  series  is  classified 
and  parallelized  with  the  Minnesota  rocks,  as  follows : 
