538  PRE-CAMRRIAN   GEOLOGY   OF    NORTH   AMERICA. 
blende,  hornblende,  sericite,  and  hydromica ;  also  diabase,  diorites, 
and  various  granites.  They  occur  in  two  large  areas  and  several 
small  ones.  One  of  the  large  areas  is  along  East  Main  River,  from 
near  the  mouth  inland  for  160  miles,  and  the  other  is  in  the  vicinity 
of  the  large  lakes  southwest  of  Lake  Mistassini. 
The  Laurentian  and  Huronian  rocks  are  overlain,  with  strong  un- 
conformity, by  a  series  of  rocks  classified  as  Cambrian,  comprising 
arkose,  sandstone,  limestone,  dolomite,  felsitic  shale,  argillite,  and 
argillaceous  shale,  together  with  gabbro,  diabase,  fine-grained  decom- 
posed traps,  and  volcanic  agglomerates.  The  fine-grained  traps  are 
interbedded  with  the  clastic  rock.  No  acidic  eruptives  appear.  On 
the  east  coast  of  Hudson  Bay  and  at  Chateau  Bay,  near  the  eastern 
entrance  of  the  Strait  of  Belle  Isle,  some  of  the  traps  have  formed 
overflows  on  the  surface  and  are  now  represented  by  dark-green,  fine- 
grained melaphyres  having  large  amygdaloidal  cavities  filled  with 
quartz  and  agate.  No  fossils  have  been  found  in  these  supposed  Cam- 
brian rocks  and  their  precise  age  and  equivalency  can  only  be  conjec- 
tured. However,  the  mode  of  occurrence  of  thick  beds  of  magnetic 
iron  ore  overlain  by  cherty,  nonfragmental  carbonates  in  this  series 
closely  resembles  that  of  the  iron  ores  of  the  Lake  Superior  region 
described  by  Irving,  Van  Hise,  and  others.  This,  with  other  char- 
acters of  resemblance,  renders  it  almost  certain  that  the  two  develop- 
ments represent  the  same  period,  or,  in  other  words,  that  the  An- 
imikie  rocks  of  Lake  Superior,  assumed  to  be  Lower  Cambrian,  are 
equivalent  to  the  rocks  here  described  as  Cambrian  in  Labrador. 
Tyrrell  and  Dowling,50  in  1897,  report  on  the  country  between 
Athabaska  Lake  and  Churchill  River  in  Canada.  The  area  covered 
by  the  report  is  bounded  on  the  south  by  Churchill  and  Clearwater 
rivers ;  on  the  west  by  the  lower  portion  of  Athabaska  River ;  on  the 
north  by  Athabaska  Lake,  Stone  River,  with  its  expansions,  Black 
and  Hatchet  lakes,  Wollaston  Lake,  and  Cochrane  or  Ice  River;  on 
the  east  by  the  lower  part  of  Cochrane  River,  Reindeer  Lake,  and 
Reindeer  River. 
Laurentian  rocks,  including  hornblende  granites,  biotite  granites, 
muscovite  granites,  granitoid  gneisses,  gabbros,  and  norites,  are  found 
outcropping  on  Churchhill  River  from  2  miles  below  the  mouth  of 
Mudjatick  River  eastward  to  the  mouth  of  Reindeer  River;  thence 
northward  they  occupy  most  of  the  eastern  part  of  the  district. 
Farther  west  they  are  followed  north  to  Cree  Lake.  In  the  northern 
part  of  the  area  they  occupy  most  of  the  northern  shores  of  Athabaska 
and  Black  lakes. 
As  far  as  at  present  known,  the  Huronian  is  represented  in  this 
district  solely  by  three  small  areas  on  the  north  shore  of  Lake  Atha- 
baska.   The  Huronian  here  includes  quartzites,  calcareous  sandstones 
