404  PRE-CAMBRTAN   GEOLOGY   OF    NORTH    AMERICA. 
slate,  and  limestone,  all  metamorphosed  by  trap  rock.  The  slate  con- 
tains some  pebbles  of  Primitive  rock,  and  thus  approaches  a  con- 
glomerate. The  limestone  at  Echo  Lake  shows  original  stratifica- 
tion, and  is  traversed  by  seams  of  hard  metamorphic  slate,  being 
nearly  in  the  condition  of  a  jasper.  They  are  undulated,  contorted, 
and  pleated  in  a  beautiful  manner. 
Channing,5  in  1847,  reports  on  an  examination  of  Sugar  Island, 
Sailors  Encampment  Island,  St.  Joseph  Island,  and  the  main  shore  to 
Sault  Ste.  Marie  on  the  American  side.  Metamorphic  sandstone 
quartz,  chlorite-slate  quartz,  feldspar  rock  quartz,  chlorite,  granite, 
and  syenite  are  all  found  and  are  constantly  intersected  by  the  trap 
dikes.  At  Echo  Lake  is  a  metamorphic  sandstone  quartz  containing 
a  stratum  of  pebbles  converted  into  jasper.  On  Sugar  Island  is  found 
metamorphic  sandstone  containing  fragments  of  metamorphic  sand- 
stone and  gray  gneiss. 
Murray,6  in  1849,  describes  the  continuation  of  his  work  on  the 
north  coast  of  Lake  Huron  west  of  French  River  and  upon  the  adja- 
cent Manitoulin  Islands.  The  pre-Potsdam  group  of  rocks  consists, 
first,  of  a  metamorphic  series,  composed  of  granitic  and  syenitic  rocks 
in  the  forms  of  gneiss,  mica  slate,  and  hornblende  slate;  and,  sec- 
ond, in  the  ascending  order,  of  a  stratified  series,  composed  of  quartz 
rock  or  sandstones,  conglomerates,  shales,  and  limestones,  with  inter- 
posed beds  of  greenstone.  The  first  of  these  series  is  in  so  highly  a 
disturbed  condition  and  is  so  much  contorted  that  it  is  impossible  to 
ascertain  its  thickness.  The  second  series  occupies  the  whole  north 
coast  of  Lake  Huron,  with  many  of  its  neighboring  islands,  between 
Little  Lake  George  and  Shebawenahning.  There  was  no  opportunity 
of  determining  the  breadth  of  country  this  series  occupies  and  the 
thickness  it  attains.  The  quartzites  sometimes  pass  into  a  sandstone, 
and  into  a  beautiful  conglomerate,  whose  pebbles  are  chiefly  of  blood- 
red  jasper.  Besides  the  jasper  conglomerates  there  are  other  con- 
glomerates, the  pebbles  and  bowlders  of  which  are  of  syenite,  varying 
from  those  of  small  size  to  those  2  feet  in  diameter;  and  these  are 
sometimes  in  a  greenish  quartz  rock  as  a  matrix  and  sometimes 
in  a  greenish  slate,  more  frequently  the  latter.  Numerous  green- 
stone dikes  traverse  the  stratified  series,  and  greenstone  masses  are 
interposed  among  the  sedimentary  beds.  On  some  small  islands 
granite  veins  and  trap  dikes  were  found  breaking  through  the  quartz 
rock,  on  one  of  which  the  latter  beds  dipped  in  opposite  directions  on 
opposite  sides  of  the  granite,  and  on  another  the  quartz  rock  was 
found  reclining  on  the  granite,  the  contact  being  seen.  The  f ossif- 
erous series  is  supported  unconformably  upon  the  older  rocks. 
Logan,7  in  1849,  next  gives  a  general  account  of  the  geology  of  the 
north  shore  of  Lake  Huron.  An  area  of  rocks  120  miles  long  and 
from  10  to  20  miles  wide  is  placed  in  a  single  formation.    This  forma- 
