450  PRE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
Limestones  are  also  found  along  Muskoka  River.  The  strata  are 
everywhere  more  or  less  corrugated,  in  many  places  exhibiting  sharp 
and  complicated  folding.  They  are  intersected  by  quartzo-felds- 
pathic  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  Mas- 
kanongi  and  Sturgeon  rivers,  its  course  being  northeasterly. 
Logan,9  in  1858,  in  describing  the  Laurentian  of  Ottawa,  thinks  it 
probable  that  it  can  be  divided  into  two  great  groups — one  character- 
ized by  the  presence  of  limestone  and  the  other  without  limestone,  and 
that  the  latter  of  these  groups  also  will  be  capable  of  subdivision. 
Often  interstratified  with  the  limestones  are  bands  of  quartzite,  which 
are  heaviest  near  the  junction  of  the  limestone  and  the  gneiss.  The 
greatest  mass  of  quartzite  is  beneath  the  limestone  and  is  400  feet 
thick.  The  limestones  of  the  Laurentian  are  influenced  in  their  strikes 
and  dips  by  subsequent  masses  of  igneous  rocks.  However,  to  these 
rocks  as  a  whole,  as  well  as  to  their  equivalent  throughout  Canada,  is 
applied  the  term  Laurentian  series,  from  the  Laurentide  Mountains, 
extending  from  Lake  Huron  to  Labrador,  which  are  composed  of  this 
rock. 
Logan,10  in  1859,  gives  an  elaborate  description  of  the  distribution 
of  the  limestones  along  Rouge  River.  Two  belts  are  found,  regarded 
as  interstratified  with  the  gneisses.  This  latter  rock  is  sometimes 
garnetiferous  and  occasionally  is  spoken  of  as  the  quartzite.  The 
total  thickness  of  rocks  exposed  on  the  Rouge  is  more  than  22,000 
feet,  of  which  more  than  5,000  is  limestone. 
Logan,11  in  1860,  finds  three  belts  of  limestone,  which  are  associated 
with  massive  orthoclase  gneiss,  mica  slate,  hornblende  rock,  and 
quartz  rock,  together  15,000  feet  thick.  The  calcareous  bands  are 
largely  associated  with  labradorite,  and  beds  of  hornblende  rock  and 
quartz  rock  are  often  thickly  studded  with  pink  garnets.  One  of  the 
beds  of  pure  white  quartz  rock  is  a  thousand  feet  thick.  Certain 
fossil-like  forms  have  been  found  which  resemble  Stromatocerium. 
The  strata  are  very  much  folded. 
Logan,12  in  1863,  gives  a  general  account  of  the  pre-Potsdam  rocks, 
which  are  called  Azoic  and  are  divided  into  the  Huronian  series  and 
the  Laurentian  system.  In  the  Laurentian  system  are  included  anor- 
thosite,  orthoclase  gneiss,  granitoid  gneiss,  quartzite,  hornblende 
schist,  mica  schist,  pyroxene  and  garnet  rocks,  and  limestones  and 
dolomites.  The  anorthosites  are  composed  of  lime-soda  feldspar, 
varying  in  composition  from  andesine  to  anorthite  and  associated 
with  pyroxene  or  hypersthene.  The  orthoclase  gneiss  has  a  never- 
failing  constancy  in  the  parallelism  of  its  mineral  constituents,  which, 
however,  is  sometimes  obscure.  This  rock  is  usually  very  feldspathic 
and  often  coarse  grained.     With  the  feldspar  and  quartz  are  often 
