LAKE    SUPERIOR   REGION.  289 
adjacent  to  thorn.  They  contain  fragments  of  hornblende  schist  and 
granite,  and  are  not  observed  to  be  cut  by  granite.  (4)  The  youngest 
rocks  are  of  Keweenawan  age.  These  cover  a  greater  area  than  do 
all  the  other  rocks  of  the  region  combined.  They  may  be  divided  into 
sediments,  basic  igneous  rocks  (dolerites),  and  acidic  igneous  rocks. 
The  sediments  are  sandstones  and  dolerites,  and  are  best  exposed  on 
the  northeast  side  of  thelake.  The  basic  igneous  rocks  have  a  greater 
development  than  any  other  single  phase.  They  range  from  fine- 
grained aphanitic  basalt  to  coarse  gabbro,  sometimes  slightly  amygda- 
loidal,  and  appear  to  intrude  the  sediments  as  sills.  The  acidic  rocks 
range  from  quartz  porphyry  to  fine-grained  granite,  and  probably 
correspond  to  the  "  red  rock  "  of  the  Keweenawan  on  the  shore  of 
Lake  Superior. 
Bain,278  in  1901,  describes  the  iron  belt  of  Lake  Nipigon,  devoting 
his  attention  mainly  to  economic  and  petrographic  features.  The 
stratigraphic  features  are  covered  in  the  general  report  of  Coleman 
summarized  on  a  previous  page. 
Coleman,279  in  1901,  reports  on  the  distribution  and  lithology  of 
iron  ranges  northeast  and  east  of  Lake  Superior.  Only  incidental 
reference  to  structural  geology  is  made. 
Coleman  and  Willmott,280  in  1901  and  1902,  describe  and  map  the 
Michipicoten  iron  ranges.    The  rocks  are  classified  as  follows: 
Laurentian Gneisses  and  granites. 
Basic  ernptives. 
Upper  Huronian ■{Acidic  ernptives. 
Dore  conglomerate. 
Eleanor   slates. 
,  Helen  iron  formation. 
Lower  Hnronian 1  TTT  .    __ 
Wawa  tnffs. 
.Gros  Cap  greenstones. 
The  Gros  Cap  greenstones  are  basic  eruptives  with  ellipsoidal 
structure,  corresponding  in  position  and  character  to  the  Ely  green- 
stones of  the  basement  complex  of  the  Vermilion  district  of  Minne- 
sota. They  are  in  part  basal  to  the  other  rocks  of  the  district,  but  in 
part  also  they  are  interbedded  with  the  rocks  of  the  Helen  iron  for- 
mation. The  Wawa  tuffs  are  acidic  schists  having  the  composition 
of  quartz  porphyry  or  felsite,  usually  in  the  form  of  tuff,  ash,  or 
breccia,  and  sometimes  show  stratification,  taken  to  indicate  depo- 
sition by  water. 
Slates  of  distinctly  sedimentary  origin,  occurring  in  thin  bands 
near  Eleanor  Lake  and  called  the  Eleanor  slates,  are  referred  to  the 
Lower  Huronian.  Their  relations  to  the  Helen  iron  formation  are 
not  known. 
The  Helen  iron  formation,  500  feel  thick,  comprises  banded  granu- 
lar silica  with  more  or  less  iron  ore,  black  slate,  siderite  with  varying 
55721— Bull.  360—09 19 
Archean < 
