254  PKE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
Fourth,  the  medial  order,  the  only  rock  of  which  is  the  Old  Red 
sandstone.  Then  follows  an  account  of  the  distribution  of  each  of 
these  orders.  The  amygdaloid  with  little  doubt  rests  upon  the  syenite 
and  granite,  although  the  junction  was  never  seen.  The  amygdaloid 
passes  into  greenstone  on  the  one  hand,  simply  by  being  divested  of 
its  nodules,  and  into  porphyry  on  the  other.  The  Old  Red  sandstone 
may  be  traced  from  one  extremity  of  the  lake  to  the  other.  Its  exist- 
ence is  noticed  on  both  shores,  and  it  is  traced  across  the  lake  by  many 
of  the  islands,  so  as  to  leave  no  doubt  of  its  being  a  general  formation 
throughout  the  whole  of  the  basin  of  Lake  Superior.  It  is  generally 
horizontally  stratified  or  nearly  so.  There  are  many  instances  of  the 
conjunction  of  the  sandstone  and  granite,  which  serve  to  prove  that 
the  sandstone  was  deposited  after  the  granite  occupied  its  present 
position.  The  sandstone,  when  conglomeratic,  as  is  very  frequently 
the  case,  contains  fragments  of  the  trap  as  well  as  of  the  inferior 
order,  which  are  rounded  by  attrition,  and  it  is  therefore  plain  that 
the  sandstone  is  later  than  the  trap  rocks.  Organic  remains  were 
sought  in  this  rock,  but  never  discovered.  It  is  placed  as  the  Old  Red 
because  of  its  position  immediately  on  the  granite,  its  structure,  and 
component  parts. 
Bayfield,218  in  1845,  places  with  the  Primary  rocks  most  of  the 
elastics  on  the  north  shore  of  Lake  Superior.  These  are  cut  by  va- 
rious greenstones.  A  red  sandstone  forms  nearly  the  entire  southern 
boundary  of  the  lake.  In  places  it  is  shattered  by  the  upheaval  of 
granite  and  by  trap  rocks  which  enter  into  the  composition  of  its  con- 
glomerates. At'  Nipigon  Bay  it  is  overlain  by  an  immense  bed  of 
greenstone.  It  is  probable,  although  not  certain,  that  this  sandstone 
underlies  the  fossiliferous  red  sandstone  of  St.  Marys. 
Logan,219  in  1847,  in  a  report  on  the  geolog}^  and  economic  minerals 
of  Lake  Superior,  gives  an  account  of  a  rather  detailed  examination 
of  the  north  shore  of  Lake  Superior  from  Pigeon  River  to  Sault  Ste. 
Marie.  Michipicoten  Island  was  also  examined.  Lake  Superior  ap- 
pears to  be  set  in  a  geological  depression,  which  presents  formations 
of  a  similar  character  on  both  the  north  and  the  south  sides,  dipping 
to  the  center.  The  series  on  the  north,  in  ascending  order,  consists  of 
the  following:  (1)  Granite  and  syenite;  (2)  gneiss;  (3)  chloritic 
and  partially  talcose  and  conglomerate  slates;  (4)  bluish  slates  or 
shales,  interstratified  with  trap;  (5)  sandstones,  limestones,  indurated 
marls,  and  conglomerates,  interstratified  with  trap.  The  gneiss  is 
succeeded  by  dark-green  slates,  which  at  the  base  appear  to  be  occa- 
sionally interstratified  with  beds  of  the  subjacent  granite  and  gneiss. 
Some  of  these  slaty  beds  have  the  quality  of  a  greenstone,  others  are 
a  mica  slate,  and  a  few  are  quartz  rocks.  Higher  in  the  series  are 
conglomerates,  the  pebbles  of  which  are  all  apparently  from  hypogene 
rocks.     Formations  4  and  5  rest  unconformably  upon  1,  2,  and  3, 
