452  PRE-CAMBRIAN    GEOLOGY   OF    NORTH   AMERICA. 
turned,  and  the  only  reliable  mode  of  working  out  the  physical  struc- 
ture is  to  continuously  follow  the  outcrop  of  each  important  mass  in 
all  its  windings  as  far  as  it  can  be  traced,  until  it  becomes  covered  by 
superior  strata,  is  cut  off  by  dislocation,  or  disappears  by  thinning 
out. 
Several  sections  are  described  in  detail.  The  general  section  is  as 
follows,  in  ascending  order:  Orthoclase  gneiss,  5,000  feet;  Trembling 
Lake  limestone,  1,500  feet;  orthoclase  gneiss,  4,000  feet;  Great 
Beaver  Lake  and  Green  Lake  crystalline  limestone,  including  inter- 
stratified  beds  of  garnetiferous  and  hornblendic  gneiss,  2,500  feet; 
orthoclase  gneiss,  the  lower  part  having  several  bands  of  quartzite, 
3,500  feet ;  Grenville  crystalline  limestone,  750  feet ;  orthoclase  gneiss, 
1,580  feet;  Proctors  Lake  limestone,  20  feet;  orthoclase  gneiss,  in- 
cluding quartzite,  3,400  feet;  anorthosite,  thickness  (wholly  conjec- 
tural) 10,000  feet;  total,  32,750  feet.  In  the  limestones  are  fossil- 
like forms  which  resemble  Stromatopora  rugosa.  Accompanying  the 
account  of  the  Laurentian  is  a  detailed  map  of  it  in  parts  of  the  coun- 
ties of  Terrebonne,  Argenteuil,  and  Two  Mountains. 
The  anorthosite  probably  overlies  the  Grenville  series  unconform- 
ably.  It  is  remarked  that  if  the  two  inferior  limestone  bands  of  the 
Grenville  series  disappear  on  reaching  the  margin  of  the  anorthosite, 
the  disappearance  is  conclusive  evidence  of  the  existence  in  the  Lau- 
rentian system  of  two  immense  sedimentary  formations,  the  one 
superimposed  unconformably  upon  the  other,  with  probably  a  great 
difference  of  time  between  them. 
Logan,12  in  1863,  first  describes  a  part  of  what  was  later  called  the 
Hastings  series.  In  ascending  order  are  found  contorted  gneiss  and 
micaceous  schists  cut  by  red  syenite  veins.  Above  this  comes  crys- 
talline limestone,  and  north  of  the  village  of  Madoc,  still  in  ascend- 
ing order,  occurs  a  somewhat  micaceous  schist,  which  contains  nu- 
merous fragments  of  rock  in  character  different  from  the  matrix,  some 
of  them  resembling  syenite  or  greenstone.  The  pebbles  are  in  places 
distinctly  rounded. 
Bigsby,13  in  1864,  states  that  crystalline  limestones  occur  in  bands 
from  50  to  1,500  feet  thick  at  Gananoque,  on  the  Lake  of  the  Thou- 
sand Islands,  and  on  the  Mattawa.  The  bands  of  marble  are  tortuous, 
and  between  them  are  sometimes  found  corrugated  seams  of  gneiss. 
Conglomerates  and  grits  occur  at  Bastard,  on  the  Ottawa,  and  at 
Madoc,  near  Lake  Ontario.  At  the  former  place,  between  the  beds  of 
marble,  is  quartzose  sandstone,  with  pebbles  of  calcareous  sandstone 
and  vitrified  quartz.  At  Madoc  village  are  interstratified  marble 
and  conglomerate,  one  being  bluish  micaceous  schist,  holding  frag- 
ments of  greenstone  and  syenite,  the  other  being  a  dolomite  with 
large  pebbles  of  quartz,  feldspar,  and  calcite.  The  occurrence  of 
limestone,  carbon,  phosphorus,  sulphur,  and  iron  ore  is  accepted  as 
