88  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
Wadsworth,  95  in  1884,  as  a  result  of  an  examination  of  a  supposed 
fossil  from  the  copper-bearing  rocks  of  lake  Superior,  described  by  Hall 
as  being  very  like  the  Huronia  or  siphuncles  of  Orthoceratites,  rinds  it 
to  be  of  inorganic  origin,  having  probably  been  formed  by  the  flowing 
of  a  pasty  lava  in  such  a  manner  as  to  raise  a  series  of  ridges,  giving  an 
appearance  closely  like  that  of  some  cephalapods.  The  interior  of  the 
specimen  is  in  all  respects  that  of  an  ordinary  volcanic  rock. 
SECTION  III.  WORK  OF  THE  MICHIGAN   GEOLOGISTS  AND  ASSOCIATES. 
Houghton,90  in  1840,  divides  the  rocks  in  the  south  and  southeastern 
part  of  the  Upper  Peninsula  into  primary  and  sedimentary.  The 
primary  region  stretches  continuously  in  a  northwestward  direction  for 
many  hundreds  of  miles,  skirting  portions  of  the  shores  of  lake  Supe- 
rior, and  constituting  the  highlands  between  that  lake  and  the  lake  of 
the  Woods.  From  these  highlands  it  stretches  a  little  east  of  lake 
Winnipeg,  far  to  the  northwest,  finally  constituting  the  immense  "  bar- 
ren grounds"  of  the  British  Possessions.  The  rocks  of  St.  Marys  river 
and  adjacent  region  comprise  greenstone,  argillite,  and  granular  quartz 
rock,  which  passes  into  an  almost  conglomeratic  quartz  rock.  In  this 
occur  small  quantities  of  hematitic  iron  ore.  The  sedimentary  rocks 
include  the  lake  Superior  sandstone  and  lime  rock  and  shales.  The 
lake  Superior  sandstone  is  nearly  continuous  on  the  southern  shore  of 
lake  Superior,  and  in  its  easterly  prolongation  rests  against  and  upon 
the  primary  range  of  St.  Marys  river,  where  it  passes  conformably 
below  the  limestone  above.  The  lake  Superior  sandstone,  in  its  east- 
erly prolongation,  does  not  attain  a  very  great  thickness,  but  in  pro- 
ceeding westerly  this  thickness  is  vastly  increased,  attaining  on  the 
south  shore  of  lake  Superior  to  several  hundred  feet.  A  careful  search 
for  fossils  in  this  sandstone  has  failed  to  reveal  a  single  one. 
Houghton,97  in  1841,  divides  the  older  rocks  of  the  upper  peninsula 
of  Michigan  into  (1)  Primary,  (2)  Trap,  (3)  Metamorphic,  (4)  Con- 
glomerate, (5)  Mixed  conglomerate  and  sandstone,  (G)  Lower  or  red 
sandstones  and  shales,  (7)  Upper  or  gray  sandstone,  The  Primary 
rocks  are  in  a  broad  sense  granite.  The  granitic  rocks  are  largely 
traversed  by  greenstone  dikes.  The  trap  rocks  of  the  district  in  a 
chronological  order  would  follow  the  metamorphic  slates  and  quartz 
rocks, %ut  the  granitic  rocks  pass  by  almost  insensible  gradation  into 
the  greenstones  of  the  trap  formation. 
The  sedimentary  rocks  on  the  south  and  southeast  of  the  main  trap 
range  are  scarcely  disturbed,  while  those  on  the  north  and  northwest- 
erly sides  are  invariably  tilted  to  a  high  angle  near  the  range  of  hills. 
The  sedimentary  rocks  on  the  north  are  traversed  by  frequent  dikes, 
varying  in  thickness  from  50  to  400  or  500  feet.  The  rocks  on  this  : 
northwestern  escarpment  were  in  an  intense  state  of  ignition  while  in 
contact  with  the  sedimentary  rocks,  as  shown  by  the  great  changes 
which  these  rocks  have  undergone.    The  author  is  disposed  to  regard 
