24  PRE-CAMBRIAN   ROCKS    OF    NORTH    AMERICA.  [bult,89. 
Logan,4  in  1854,  applies  to  the  series  before  called  "  mctamorphic " 
underlying  the  fossiliferous  formations  of  Canada  the  name  Laurentian 
series,  because  metamorphic  is  applicable  to  any  series  of  altered  rocks. 
The  proposed  name  is  founded  on  that  given  by  Mr.  Garneau  to  the 
chain  of  hills  which  they  compose.  Above  the  Laurentian  series  is  the 
Potsdam  sandstone. 
Murray,5  in  1854,  remarks  that  south  of  the  Laurentian  series  are 
the  more  recent  fossiliferous  rocks.  The  Laurentian  series  consists  of 
masses  of  micaceous  and  hornblende-gneiss  and  masses  of  interstrati- 
fied  crystalline  limestone.  Intrusive  granite  is  found  in  the  gneiss. 
The  magnesian  crystalline  limestone  layers,  one  700  feet  thick,  are  per- 
sistent. A  section  of  gneiss,  mica-schist  and  quartzite,  all  sometimes 
garnetiferous,  and  two  belts  of  limestone,  together  140  feet  thick,  make 
up  a  succession  1,369  feet  thick  at  Birch  lake.  In  the  series  is  a  con- 
glomerate, the  matrix  of  which  in  one  case  is  a  limestone  and  the  peb- 
bles of  quartz  and  feldspar.  In  another  conglomerate  are  distinct  peb- 
bles in  a  talcose  slaty  matrix.  These  pebbles  are  sometimes  distinctly 
rounded  and  flattened,  the  flat  sides  always  lying  parallel  with  the  bed- 
ding. The  pebbles  vary  from  5  or  6  inches  in  diameter  to  those  so  small 
as  not  to  exceed  the  size  of  snipe  shot. 
Logan,6  in  1857,  describes  the  Laurentian  formation  for  some  dis- 
tance north  of  the  Ottawa  river  between  rivers  Eouge  and  du  Nord. 
The  rocks  are  found  to  be  limestone,  gneiss,  and  quartzite.  The  lime- 
stone formations  are  used  chiefly  in  working  out  the  structure,  but  even 
with  this  guide,  on  account  of  the  repetition  of  layers  by  folding  and 
lack  of  fossils,  the  work  is  very  difficult.  All  of  the  above  rocks  are 
taken  to  be  metamorphosed  sediments.  They  are  cut  by  eruptives,  such 
as  syenite,  porphyry,  and  greenstone,  which  are  older  than  the  fossil- 
iferous formations. 
Murray,7  in  1857,  finds  Laurentian  rocks  largely  exposed  between 
Georgian  bay  and  the  Ottawa  river.  The  rocks  are  red  and  gray 
gneisses,  micaceous  and  hornblendic  schists,  quartzite,  and  crystalline 
limestone.  On  lake  Mpissing  and  its  islands  is  found  the  Laurentian 
formation,  consisting  of  gneiss,  mica-schist,  hornblende  schist,  crystal- 
line limestone,  and  associated  with  this  beds  of  specular  ore.  Lime- 
stones are  also  found  along  Muskoka  river.  The  strata  are  everywhere 
more  or  less  corrugated,  in  many  places  exhibiting  sharp  and  compli- 
cated folding.  They  are  intersected  by  quartzo-feldspathic  and  quartz 
veins.  The  Laurentian  rocks  of  Georgian  bay  are  separated  from  the 
Huronian  north  of  lake  Huron  by  a  line  running  from  the  northwest 
side  of  Shibahahnahning  to  the  junction  of  the  Maskanongi  and  Stur- 
geon rivers,  its  course  being  in  a  northeasterly  direction. 
Hunt,8  in  1857,  states  that  stratified  feldspathic  rocks  are  closely  as- 
sociated with  the  crystalline  limestones,  which  alternate  with  gneissoid 
and  quartzose  rocks  of  the  Laurentide  mountains.  These  rocks,  besides 
containing  pyroxene,which  passes  over  into  hypersthene  and  a  triclinic 
feldspar,  contain  as  accidental  minerals,  mica,  garnet,  and  ilweuite. 
