426  PKE-C AMEBIAN   GEOLOGY   OF   NOETH   AMEEICA. 
quarry  north  of  Garden  River,  the  unconformity  between  the  upper 
slate  conglomerate  and  the  limestone  was  again  examined.  At  Echo 
Lake,  where  Logan  had  mapped  a  southward-dipping  monoclinal 
succession  of  lower  slate  conglomerate,  limestone,  and  upper  slate 
conglomerate,  the  succession  was  found  to  be :  Quartzite,  constituting 
the  thickest  formation  above  the  lake,  grading  upward  through  gray- 
wacke  into  thin  conglomerate  not  more  than  30  feet  thick,  containing 
granite  and  green  schist  fragments,  this  in  turn  into  limestone  not 
more  than  50  feet  thick,  and  this  overlain  by  the  upper  slate  con- 
glomerate in  considerable  thickness.  This  upper  slate  conglomerate 
bears  many  well-rounded  fragments  of  the  limestone  of  varying  sizes, 
showing  a  marked  unconformity  between  the  two  formations.  The 
limestone,  instead  of  occurring  in  a  single  east-west  band  with  south- 
erly dip,  is  in  a  series  of  synclinal  patches,  and  the  conglomerate  both 
to  the  north  and  to  the  south  is  the  thin  lower  slate  conglomerate 
brought  up  by  the  synclinal  structure. 
From  Echo  Lake  southeastward  to  Thessalon  the  mapping  of 
Logan  and  Murray  was  found  to  be  substantially  correct.  Certain 
features  in  the  vicinity  of  the  Bruce  mines  suggest  that  there  may  be 
an  unconformity  above  the  slate  conglomerate,  but  these  are  not 
decisive. 
The  estimate  of  the  thickness  of  the  Huronian,  18,000  feet,  by 
Logan  and  Murray  is  far  too  large,  the  true  thickness  probably  not 
exceeding  12,000  feet. 
The  contact  of  Logan's  gray  quartzite  and  granite  to  the  east  of 
Thessalon  was  again  examined  and  the  conclusion  of  previous  years, 
that  the  conglomerate,  resting  unconformably  upon  the  granite,  pre- 
sumably represents  the  base  of  the  Huronian  series,  was  confirmed. 
The  possibility  having  suggested  itself  that  this  conglomerate  might 
represent  the  base  of  an  upper  series  of  the  district,  the  area  to  the 
northwest  was  studied  and  the  conclusion  was  reached  that  the  con- 
glomerate is  at  the  base  of  the  Huronian  of  the  district,  as  mapped 
by  Logan  and  Murray. 
Miller,48  in  1903,  gives  a  resume  of  the  occurrence  of  iron  ore  in 
northern  Ontario  and  incidentally  discusses  their  geological  relations. 
Coleman,49  in  1903,  describes  and  maps  the  nickel  deposits  near 
Sudbury,  Ontario,  and  incidentally  discusses  the  geology  of  the  re- 
gion. The  probable  succession  and  age  of  the  rocks  of  the  district  are 
as  follows,  in  ascending  order: 
{Dikes  of  diabase. 
Younger  granite. 
Nickel-bearing  eruptive ;  norite ;  micropegmatite ;  granite. 
Animikie  (?)  or  Upper  Huronian  (  ?) — Oval  area  of  tuffs,  sandstones,  and  slates, 
overlying  the  preceding. 
Laurentian — Granitoid  gneiss. 
Upper  Huronian  [Green  schists  and  greenstones. 
lArkoses,  quaitzites,  and  graywackes. 
