AREA    NORTH   AND   NORTHEAST   OF   LAKE    HURON.  413 
laceous  sandstones  or  graywackes,  interstratified  with  slaty  belts  and 
overlain  at  the  summit  by  black  slates.  The  stratified  Huronian 
rocks,  as  well  as  the  gneiss  and  quartz  syenite,  are  traversed  by  dikes 
of  coarsely  crystalline  diabase,  which  are  often  large  and  can  be 
traced  for  considerable  distances. 
Winchell  (N.  H.),29  in  1891,  gives  further  observations  upon  the 
Huronian.  Northwest  of  Sudbury  and  eastward  from  Algoma  there 
are  two  formations.  In  both,  the  slate  and  slate  conglomerate  con- 
stitute the  upper  formation.  In  the  Sudbury  region  the  underlying 
rocks  are  largely  felsitic,  but  are  also  occasionally  micaceous  and 
hornblendic.  In  the  section  eastward  from  Algoma  the  underlying 
formation  seems  to  be  the  Mississagui  quartzite,  with  interbedded 
green  fissile  schist,  the  mica  schist  varying  into  hornblende  schist. 
Logan's  Mississagui  quartzite  is  supposed  not  to  be  Logan's  lowest 
gray  quartzite,  but  is  probably  a  constituent  part  of  the  Keewatin. 
It  is  concluded  that  the  observations  confirm,  or  at  least  do  not  con- 
travene, the  conclusion  that  the  Huronian  system  of  the  Canadian 
reports  embraces  two  or  three  formations,  one  of  these  being  the  true 
Huronian  first  described  and  mapped  by  Murray,  another  the  Kee- 
watin of  Lawson,  and  another  the  series  of  crystalline  schists  styled 
the  Vermilion  series. 
Pumpelly  and  Van  Hise,  30  in  1892,  describe  the  relations  of  the 
Huronian  and  Laurentian,  and  also  give  evidence  for  the  divisibility 
of  the  Huronian  into  two  series  as  advocated  by  Winchell. 
In  reference  to  the  latter  point,  at  a  limestone  quarry  about  2  miles 
northeast  of  Garden  River  the  upper  slate  conglomerate  was  found 
in  actual  contact  with  the  limestone  member.  This  conglomerate  has 
a  rough  appearance  of  stratification  and  bears  numerous  fragments  of 
limestone,  many  of  them  more  than  a  foot  in  length  and  all  in  pre- 
cisely the  condition  in  which  the  original  limestone  is  now.  In  this 
conglomerate  are  also  numerous  fragments  of  schist  and  granite. 
The  line  of  contact  could  be  traced  only  a  short  distance,  and  it  ap- 
pears to  follow  somewhat  closely  the  lamination  of  the  limestone. 
These  relations  clearly  indicate  that  after  the  limestone  was  depos- 
ited, before  the  beginning  of  the  time  of  the  upper  slate  conglomer- 
ate, there  was  a  considerable  interval  of  erosion.  The  observations 
thus  tend  to  confirm  Winchell's  conclusion  that  the  Laurentian  is 
divisible  into  two  discordant  series,  the  break  occurring  above  the 
lower  limestone.  If  this  break  shall  prove  to  be  general  at  this  hori- 
zon, it  places  in  the  Lower  Huronian,  using  Logan's  thicknesses  for 
the  formations,  about  5,000  feet,  and  in  the  Upper  Huronian  about 
13,000  feet. 
Almost  immediately  below  the  limestone  was  found  the  lower  slate 
conglomerate,  which  in  lithological  character  is  precisely  like  the  slate 
conglomerate  in  contact  with  the  granite  belowT  described. 
