68  PKE-CAMBKIAN    GEOLOGY    OF    NOETH    AMERICA. 
Nebraska,  part  of  the  Ozark  Mountains  in  Arkansas,  and  in  northern 
New  Jersey  Azoic  gneiss,  limestone,  and  other  crystalline  rocks  con- 
taining beds  of  iron  ore.  The  rocks  of  the  Azoic  are  mostly  of  the 
metamorphic  series,  related  to  granite,  gneiss,  syenite,  and  the  like, 
but  they  embrace  only  the  most  ancient  of  these  rocks.  The  Azoic 
rocks  are  nearly  all  crystalline,  a  few  sandstones,  slates,  and  con- 
glomerates being  the  only  exceptions.  They  are  remarkable  for  the 
small  amount  of  silica  they  contain,  as  shown  in  the  diorites  and  lab- 
radorite  rocks.  Prevalence  of  iron  ore  is  another  characteristic,  and 
none  of  the  minerals  are  simple  silicates  of  aluminum.  While  the 
Azoic  rocks  are  crystalline  the}7  follow  one  another  in  variations  and 
alternations,  like  sedimentary  beds  of  later  date.  Granite  or  gneiss 
may  lie  between  layers  of  slate  or  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,  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  lime- 
stone strata,  the  occurrence  of  graphite  in  the  limestone,  and  the  oc- 
currence 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  have  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 
Canada.  He  states  that  the  rocks  composing  the  Laurentide  Moun- 
tains in  Canada  and  the  Adirondacks  in  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,  alumi- 
nous, 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  un- 
conformably  upon  the  Lower  (Laurentian)  series.  The  united  thick- 
ness of  these  two  groups  in  Canada  can  not  be  less  than,  and  probably 
much  exceeds,  30,000  feet.  A  third  Canadian  group,  the  Huronian, 
has  been  shown  by  Murray  to  be  about  18y000  feet  thick,  and  to  con- 
sist chiefly  of  quartzites,  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  believed  to  be  more  recent  than  the  Upper  Lauren- 
tian series,  although  the  two  formations  have  never  yet  been  seen  in 
contact. 
