36  PRE-CAMBRIAN   ROCKS    OF    NORTH   AMERICA.  [bull. 86. 
by  seams  of  hard  inetamorphic  slate,  being  nearly  in  the  condition  of 
a  jasper.  They  are  undulated,  contorted,  and  pleated  in  a  beautiful 
manner. 
Chaining,35  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, 
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  metamor- 
phic sandstone,  containing  fragments  of  metamorphic  sandstone  and 
gray  gneiss. 
Murray,36  in  1849,  describes  the  continuation  of  his  work  on  the 
north  coast  of  lake  Huron  west  of  French  river,  and  upon  the  adjacent 
Manitoulin  islands.  The  pre-Potsdam  group  of  rocks  consist,  firstly, 
of  a  metamorphic  series,  composed  of  granitic  and  syenitic  rocks  in 
the  forms  of  gneiss,  mica-slate,  and  hornblende-slate;  and,  secondly,  in 
ascending  order,  of  a  stratified  series,  composed  of  quartz  rock  or  sand- 
stones, conglomerates,  shales,  and  limestones,  with  interposed  beds  of 
greenstone.  The  first  of  these  series  is  in  so  highly  a  disturbed  con- 
dition, 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.  The  breadth  of  country  this  series 
occupies,  and  the  thickness  it  attains,  there  was  no  opportunity  of 
determining.  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  conglomer- 
ates, the  pebbles  and  bowlders  of  which  are  of  syenite,  varying  from 
those  of  small  size  to  those  2  feet  in  diameter ;  and  these  are  some- 
times 'in  a  greenish  quartz  rock  as  a  matrix,  and  sometimes  in  a 
greenish  slate,  more  frequently  the  latter.  Numerous  greenstone  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  fossiliferous  series  is  supported 
unconformably  upon  the  older  rocks. 
Logan,37  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  formation 
rests  unconformably  below  the  Silurian,  as  shown  by  the  fact  that  the 
latter  horizontal  strata  rest  upon  the  uptilted  edges  of  the  quartz  rock, 
fill  the  valleys  between,  and  overtop  the  mountains.  Upon  account  of 
the  eruptive  material  which  the  formation  contains  it  is  placed  as  the 
