444       PRE- CAMBRIAN  HOCKS  OF  NORTH  AMERICA.     |but.i..s<3. 
schist,  and  quartz-rock  may  have  any  place  in  the  series.  The  Azoic 
rocks  are  the  results  of  alteration  of  sedimentary  strata,  as  is  shown 
by  the  fact  that  schists  graduate  into  true  slates,  and  quartzites  into 
sandstones,  and  conglomerates  and  gneiss  into  gneissoid  granite,  and 
thence  to  true  granite  and  syenite.  As  evidence  of  life  in  the  Azoic 
age  are  cited  the  formations  of  limestone  strata,  the  occurrence  of 
graphite  in  the  limestone,  the  occurrence  of  anthracite  in  small  pieces 
in  the  iron-bearing  rocks  of  Arendal,  Norway.  Crystalline  rocks  have 
been  formed  in  various  ages,  those  in  New  England,  for  instance,  long- 
after  the  Azoic;  hence  it  is  possible  that  some  of  the  Azoic  rocks  lia  vo 
undergone  a  second  or  third  alteration  subsequent  to  the  original  one 
in  the  Azoic  age. 
Logan,6  in  1864,  gives  a  general  account  of  the  ancient  rocks  of  Can- 
ada. He  states  that  the  rocks  composing  the  Laurentide  mountains  in 
Canada  and  the  Adirondacks  in  the  State  of  New  York  are  the  oldest 
in  North  America.  They  have  been  shown  to  be  a  great  series  of  strata 
which,  though  profoundly  altered,  consist  chiefly  of  quartzose,  alumin- 
ous and  argillaceous  rocks,  like  the  sedimentary  deposits  of  less  ancient 
times.  This  great  mass  of  crystalline  rocks  is  divided  into  two  groups 
and  it  appears  that  the  Upper  (Labradorian)  rests  unconformably  upon 
the  Lower  (Laurentian)  series.  The  united  thickness  of  these  two 
groups  in  Canada  can  not  be  less  than  30,000  feet,  and  probably  much 
exceeds  it.  A  third  Canadian  group,  the  Huronian,  has  been  shown  by 
Murray  to  be  about  18,000  feet  thick,  and  to  consist  chiefly  of  quartz- 
ites, slate  conglomerates,  diorites,  and  limestones.  The  horizontal 
strata,  which  form  the  base  of  the  Lower  Silurian  in  western  Canada, 
rest  upon  the  upturned  edges  of  the  Huronian  series,  which,  in  its  turn, 
unconformably  overlies  the  Lower  Laurentian.  The  Huronian  is  be- 
lieved to  be  more  recent  than  the  Upper  Laurentian  series,  although 
the  two  formations  have  never  yet  been  seen  in  contact. 
The  united  thickness  of  these  three  great  series  may  possibly  far 
surpass  that  of  all  the  succeeding  rocks,  from  the  base  of  the  Paleozoic 
series  to  the  present  time.  We  are  thus  carried  back  to  a  period  so 
remote  that  the  appearance  of  the  so-called  Primordial  fauna  maybe  con- 
sidered a  comparatively  modern  event.  We,  however,  find  that,  even 
during  the  Laurentian  period,  the  same  chemical  and  mechanical  proc- 
esses which  have  ever  since  been  at  work  disintegrating  and  recon- 
structing the  earth's  crust  were  in  operation  as  now.  In  the  con- 
glomerates of  the  Huronian  series  there  are  inclosed  bowlders  derived 
from  the  Laurentian,  which  seem  to  show  that  the  parent  rock  was 
altered  to  its  present  crystalline  condition  before  the  deposition  of  the 
newer  formation,  while  inter  stratified  with  the  Laurentian  limestones 
there  are  beds  of  conglomerate,  the  pebbles  of  which  are  themselves 
rolled  fragments  of  still  older  laminated  sand-rock,  and  the  formation 
of  these  beds  leads  us  still  further  into  the  past. 
In  both  the  Upper  and  Lower  Laurentian  series  there  are  several 
