26  PRE-CAMBRIAN   ROCKS    OF   NORTH    AMERICA.  [bull.86. 
netite,  is  interstratified  with  or  not  far  removed  from  the  limestone 
bands.  Associated  with  the  limestones  are  dolomites,  which,  however, 
compose  distinct  beds. 
There  is  not  any  special  order  in  the  masses,  but  beds  of  hornblende 
rock  and  hornblende  schists  are  more  abundant  near  the  inter  stratified 
bands  of  limestone  than  elsewhere,  and  in  the  same  neighborhood  there 
usually  occurs  a  more  frequent  repetition  of  beds  of  quartzite  than  in 
other  parts.  Garnet  is  sometimes  disseminated  in  the  micaceous  and 
hornblendic  gneiss  and  quartzite  and  are  commonly  confined  to  the 
immediate  proximity  of  the  limestones.  The  limestones  and  gneiss 
beds  as  a  whole  are  generally  conformable  to  them.  It  often  happens 
that  a  subordinate  layer  of  gneiss  will  display  contortions  of  the  most 
complicated  description.  Notwithstanding  the  highly  crystalline  con- 
dition of  the  Laurentian  rocks,  beds  of  unmistakable  conglomeritic 
character  are  occasionally  met  with.  These  generally  occur  in  the 
quartzite  or  micaceous  beds.  The  intrusives  of  the  Laurentian  consist 
chiefly  of  syenite  and  greenstones.  The  greenstone  dikes  are  always 
interrupted  by  the  syenite  when  they  have  been  found  to  come  in  con- 
tact with  it,  and  the  latter  is  therefore  of  posterior  date.  A  mass  of 
intrusive  syenite'  occupies  an  area  of  about  36  square  miles  in  the 
townships  of  Grenville,  Chatham,  and  Wentworth.  It  is  cut  and  pen- 
etrated by  masses  of  a  porphyritic  character  which  are  therefore  of  a 
still  later  date. 
The  Laurentian  series  stretches  on  the  north  side  of  the  St.  Lawrence 
from  Labrador  to  Lake  Huron,  and  occupies  by  far  the  larger  portion 
of  Canada.  Its  strata  probably  possesses  a  very  great  thickness.  To 
determine  the  superposition  of  the  various  members  of  such  an  ancient 
series  is  a  task  which  has  never  yet  been  accomplished,  and  the  diffi- 
culties attending  it  arise  from  the  absence  of  fossils  to  characterize  its 
different  members.  Bands  of  crystalline  limestones  are  easily  distin- 
guished from  the  bands  of  gneiss,  but  it  is  scarcely  possible  to  know 
from  local  inspection  whether  any  mass  of  limestone  in  one  part  is 
equivalent  to  a  certain  mass  in  another  part.  They  all  resemble  one 
another  lithologically.  The  dips  avail  but  little  in  tracing  out  the  struc- 
ture, for  in  numerous  folds  in  the  series  the  dips  are  overturned,  and  the 
only  reliable  mode  of  working  out  the  physical  structure  is  to  continu- 
ously 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  inters tratified  beds 
of  garnetiferous  and  hornblendic  gneiss,  2,500  feet;  orthoclase-gneiss, 
the  lower  part  having  several  bands  of  quartzite,  3,500  feet;  Grenville 
crystalline  limestoue,  750  feet;  orthoclase  gueiss,  1,580  feet;   Proctor's 
