230  PRE-CAMBEIAN    GEOLOGY    OF    NORTH   AMERICA. 
est  igneous  rocks  are  greenstones.  These  vary  from  massive  to  schis- 
tose, and  even  to  conglomerate-breccias.  The  acidic  rocks  were  in- 
truded later  than  the  basic  rocks.  They  were  originally  for  the  most 
part  quartz  porphyries,  but  these  have  been  extensively  changed  to 
sericite  schists  and  conglomerate-breccias,  and  to  rocks  intermediate 
between  these  and  the  original  form.  Within  the  larger  masses  of  the 
igneous  rocks,  both  basic  and  acidic,  are  frequently  included  frag- 
ments from  both  the  slate  and  iron  formations,  from  those  of  small 
size  to  masses  more  than  100  feet  long. 
The  conglomerate-breccias  are  of  dynamic  origin.  The  first  step 
in  the  development  of  the  breccias  was  the  formation  of  two  intersect- 
ing sets  of  planes  of  fracture,  dividing  the  originally  massive  rocks 
into  roughly  rhomboidal  blocks.  Their  further  development  de- 
pended on  continued  movement  between  these  blocks  under  pressure, 
which  resulted  in  enlarging  the  shearing  zones  at  the  surfaces  of  con- 
tact and  rounding  the  angles.  The  slate  and  jasper  inclusions  origi- 
nally plucked  off  from  the  rocks  which  the  porphyries  and  green- 
stones invaded,  shared,  of  course,  the  subsequent  history  of  their 
captors.  The  fact  that  the  jasper  inclusions  are  frequently  rounded, 
while  those  of  slate  are  not,  is  explained  by  the  difference  in  the  elas- 
ticity of  the  two  rocks.  The  slate  inclusions  readily  yielded  and 
finally  took  a  permanent  set  under  the  deforming  forces,  while  the 
harder  and  more  rigid  jasper,  in  fragments  of  limited  size  and  diverse 
orientation,  behaved  like  the  inclosing  porphyry.  The  boundaries  of 
the  inclusions  were  generally  the  surfaces  along  which  rupture  took 
place,  although,  as  has  already  been  said,  jasper  in  a  few  instances  is 
found  partly  held  in  porphyry  inclusions. 
As  to  structure,  the  main  slate  area  is  anticlinal;  both  north  and 
south  of  this  area  the  jasper  succeeds  the  slates.  The  southern  jasper 
continues  in  a  complex  syncline,  and  south  of  this  is  found  the  north- 
ern limb  of  another  anticline  of  slates,  the  southern  limb  not  being 
exposed.  Still  farther  south  is  the  jasper  of  Lee  and  Tower  hills, 
which  appears  to  form  the  southern  and  western  edges  of  a  complex 
syncline.     All  of  these  folds  pitch  toward  the  east. 
The  ore  deposits  are  found  to  conform  in  occurrence  to  the  laws 
worked  out  by  Van  Hise  in  reference  to  other'  districts  of  the  Lake 
Superior  region;  that  is,  (1)  they  occur  for  the  most  part  in  pitching 
troughs  with  impervious  basements.  Usually  this  impervious  base- 
ment is  one  or  more  of  the  different  varieties  of  the  eruptive  rocks. 
(2)  They  are  secondary  concentrations  produced  b}r  downward-per- 
colating waters,  the  silica  being  leached  out  and  the  iron  ore  deposited. 
Winchell  (Alexander)  ,195  in  1897,  gives  a  detailed  petrographical 
description  of  the  Koochiching  granite  occurring  on  the  north  bound- 
ary of  Minnesota,  about  2  miles  west  of  Rainy  Lake.     The  rock  is  a 
