LAKE    SUPEEIOR   REGION.  117 
trappean  bands  of  Keweenaw  Point  bear  to  the  conglomerates.  Many 
of  the  slates  appear  to  be  composed  of  pulverulent  greenstone,  as 
though  they  might  originally  have  been  ejected  as  an  ash  and  subse- 
quently deposited  as  a  sediment.  They  pass  by  imperceptible  grada- 
tions from  a  highly  fissile  to  a  highly  compact  slate. 
In  sec.  19,  T.  49  N.,  R.  27  W.,  are  found  a  talcose  and  chlorite 
slate  and  quartzose  rocks  enveloping  pebbles  and  displaying  obscure 
lines  of  stratification.  In  sec.  32,  T.  48  N.,  R.  26  W.,  and  near 
Jackson  Company's  forge  are  found  quartzose  conglomerates.  On  the 
line  between  sees.  29  and  32,  T.  47  N.,  R.  27  W.,  is  a  conglomerate 
forming  an  isolated  rounded  elevation,  which  is  made  up  of  coarse 
blocks  of  various  sorts  belonging  to  the  neighboring  trappean  and 
slaty  beds.  Among  them  are  found  fragments  of  the  rock  associated 
with  iron,  and  masses  of  the  iron  itself,  and  of  the  banded  and  jaspery 
varieties.  Most  of  the  fragments  of  the  breccia  are  but  slightly 
rounded  and  worn  on  their  edges,  having  in  this  respect  the  appear- 
ance of  a  friction  conglomerate.  The  blocks  are  cemented  together 
by  a  hard  ferruginous  paste. 
The  granites  belong  to  two  epochs :  The  granite  of  the  northwest- 
ern coast  and  the  vicinity  of  Pigeon  River  was  elevated  before  the 
Azoic  period,  since  the  upper  portion  of  the  slates  repose  horizontally 
around  it;  while  that  of  the  northeastern  coast  and  that  which. forms 
the  axis  of  the  river  systems  of  the  two  lakes  were  elevated  after  the 
termination  of  the  Azoic  period  and  before  the  dawn  of  the  Silurian, 
since  the  granite  has  disturbed  the  upper  beds  of  slate,  while  the 
lower  beds  of  the  Potsdam  rest  undisturbed  around  it. 
The  masses  of  iron  ore  and  jasper  have  none  of  the  characteristics  of 
vein  deposits.  They  are  intercalated  among  the  metamorphosed  sedi- 
mentaries  and  have  an  intimate  association  with  the  trappean,  por- 
phyritic,  and  serpentine  rocks.  If  the  trappean  rocks  were  an  invari- 
able accompaniment  the  ores  would  with  little  hesitancy  all  be  as- 
signed to  a  purely  eruptive  origin ;  but  when  they  are  found  in  the 
form  of  beds,  in  clearly  metamorphic  strata,  having  a  common  bear- 
ing and  inclination,  they  are  regarded  as  having  been  derived  from 
the  destruction  of  previously  formed  igneous  masses,  and  their  pres- 
ent association  as  having  resulted  from  aqueous  deposition.  The 
Azoic  period  having  been  one  of  long-continued  and  violent  mechan- 
ical action,  there  is  no  reason  to  doubt  that  many  of  the  strata  of 
which  it  is  composed  may  have  been  derived  from  the  ruins  of  pre- 
viously formed  rocks  of  the  same  age,  both  sedimentary  and  igneous, 
as  is  shown  by  the  case  of  the  knob  of  conglomerate  already  men- 
tioned. The  minute  banding  of  the  ore  and  jasper  can  hardly  be 
explained  by  any  other  than  the  action  of  segregating  force-  in  an 
igneous  rock.  The  authors  are  then  disposed  to  regard  the  specular 
and  magnetic  oxide  of  iron  as  a  purely  igneous  product,   in  some 
