AREA   NORTH   AND   NORTHEAST   OF   LAKE    HURON.  405 
tion  rests  imconformably  below  the  Silurian,  as  shown  by  the  fact  that 
the  latter  horizontal  strata  rest  upon,  the  uptilted  edges  of  the  quartz 
rock,  fill  the  valleys  between,  and  overtop  the  mountains.  On  account 
of  the  eruptive  material  which  the  formation  contains  it  is  placed  as 
the  probable  equivalent  of  the  copper-bearing  group  of  Lake  Supe- 
rior. The  series  is  divided  into  rocks  of  sedimentary  and  rocks  of 
eruptive  origin.  The  sedimentaries  consist  of  sandstones,  conglom- 
erates, slates,  and  limestones.  The  greenstones  are  of  igneous  origin, 
and  are  of  two  classes,  intrusives  and  overflows.  The  intrusives  are 
in  part  as  sheets  and  in  part  as  dikes.  The  various  kinds  of  sedi- 
mentary beds  grade  into  each  other,  while  the  greenstones  do  not  thus 
grade  into  the  sediments,  and  therefore  present  a  strong  contrast  to 
the  real  sedimentary  beds.  The  dikes  and  overflow  sheets  are  litho- 
logically  alike,  and  the  dikes  reveal  a  history  which  has  two  or  three 
episodes.  The  chief  difference  in  the  copper-bearing  rock  of  Lakes 
Huron  and  Superior  seems  to  lie  in  the  great  amount  of  amygda- 
loidal  trap  in  the  latter  and  of  white  quartz  or  sandstone  in  the  for- 
mer ;  but  there  are  strong  points  of  resemblance ;  so  it  is  highly  prob- 
able, if  not  almost  certain,  that  they  are  equivalent  and  beneath  the 
lowest  fossiliferous  deposits.  On  the  east  and  west  the  series  seems 
to  repose  on  granite. 
Murray,8  in  1850,  gives  the  result  of  a  survey  of  Spanish  River. 
On  this  stream  he  finds  exposed  a  granitic  or  metamorphic  group  and 
a  quartz-rock  group.  The  latter  contains  quartzites,  slates,  and  con- 
glomerates, holding  sometimes  pebbles  of  jasper,  but  more  often  of 
syenite  or  granite,  as  well  as  limestones  and  dikes  and  beds  of  intru- 
sive greenstones,  and  can  scarcely  be  less  than  10,000  feet  thick.  The 
granitic  group  appears  to  rise  from  beneath  the  metamorphic  group 
at  two  places. 
Logan,9  in  1852,  states  that  on  Lake  Huron  the  Lower  Silurian 
group  rests  unconformably  upon  a  siliceous  series  that  contains  one 
band  of  limestone,  about  150  feet  in  thickness,  having  leaves  of  chert, 
but  without  discovered  fossils.  The  series  includes  the  copper-bear- 
ing rocks  of  that  district,  is  interstratified  with  igneous  masses,  and 
has  a  thickness  of  at  least  10,000  feet;  it  is  supposed  to  bo  of  the  Cam- 
brian epoch.  The  gneissoid  group  is  probably  still  older,  and  its  .con- 
dition is  such  as  to  make  it  reasonable  to  suppose  that  it  consists  of 
altered  aqueous  deposits.  e 
Murray,10  in  1857,  describes  several  of  the  more  important  streams 
between  Georgian  Ray  and  Lake  Nipissing.  They  embrace  two  of 
the  oldest  recognized  geological  formations,  the  Laurentian  and  the 
lluronian;  the  rocks  of  the  latter  are  more  recent  and  have  been  ob- 
served to  pass  unconformably  below  the  lowest  of  the  fossiliferous 
strata  of  the  Silurian  system.  The  contorted  gneiss  of  the  Lauren- 
tian series,  with  its  associated  micaceous  and  hornblendic  schists, 
