458  PRE -CAMBRIAN    ROCKS    OF- NORTH    AMERICA.  [bull.  86. 
strings  and  masses  composed  of  iron  ore,  bisilicates,  and  mica,  as  well 
as  larger  porphyritic  crystals  of  plagioclase.  Even  when  tolerably  con- 
stant in  composition  there  is  generally  a  great  variation  in  size  of  grain, 
coarse  and  fine  layers  alternating  in  rude  bands  or  rounded  masses. 
In  the  case  of  some  of  the  areas  there  can  be  but  little  doubt  that  the 
anorthosite  is  eruptive;  in  others,  however,  it  seems  to  be  interstrati- 
fied  with  the  Laurentian  gneiss,  and  in  one  of  them  to  merge  imper- 
ceptibly into  it.  The  original  relations  of  the  rocks  are,  of  course, 
much  obscured  by  the  effects  of  subsequent  heat  and  pressure.  The 
evidence  at  present,  however,  seems  to  indicate  that  the  anorthosites 
are  the  result  of  some  kind  of  extravasation  which  in  early  times  corre- 
sponded to  what  in  modern  times  we  call  volcanic  eruption. 
Dawson,  (Sir  William),17  in  1888,  describes  the  Eozoic  rocks  of  the 
Atlantic  coast  and  compares  them  with  those  of  western  Europe  and 
the  interior  of  America. 
The  Laurentian  system  consists  in  all  parts  of  the  world  largely  01  j 
orthoclase-gneiss  associated  with  crystalline  schists,  and  locally  quartz- 
ites  and  limestone. 
No  one  who  has  studied  the  typical  districts  of  the  Ottawa  river  can] 
doubt  for  a  moment  that  they  are  regularly  bedded  deposits,  and  that 
in  the  Middle  Laurentian  those  conditions  which  in  later  periods  have i 
produced  beds  of  limestone,  sandstone,  iron  ore,  and  even  of  coal,  were  j 
already  in  operation  on  a  gigantic  scale.  At  the  same  time  it  may  bej 
admitted  that  some  areas  of  the  lower  gneiss  may  be  cooled  portions  i 
of  an  original  igneous  mass,  and  that  many  of  the  schistose  rocks  may  5 
be  really  bedded  igneous  materials. 
Laurentian  rocks  compose  the  nucleus  of  the  island  of  Newfound-^ 
land,  occur  in  cape  Breton,  and  in  southern  New  Brunswick. 
In  the  typical  area  of  lake  Huron,  as  originally  described  by  Logan  | 
and  Murray,  the  Huronian  rests  unconformably  on  the  Lower  and 
Middle  Laurentian,  and  presents  a  great  contrast  in  point  of  mineral 
character  to  these  formations.  It  is  comparatively  little  disturbed, 
and  is  clastic  rather  than  crystalline  in  character.  This  point  has  been* 
well  insisted  upon  by  Bonney  and  by  Irving  in  recent  papers.  Further,, 
its  conglomerates  contain  pebbles  of  Laurentian  rock  in  the  same 
crystalline  state  in  which  these  rocks  are  found  at  present.  It  consists! 
chiefly  of  quartzites,  conglomerates  of  different  kinds,  limestone,  ana 
slates,  sometimes  chloritic,  with  interbedded  diorite. 
In  Newfoundland  the  older  slate  series  of  Jukes  is  lithologically  very 
like  the  Huronian,  and  this  likeness  is  increased  by  the  fact  that  red 
sandstones  and  conglomerates  like  the  Keweenian  of  the  West  over- 
lie these  lower  slates. 
On  the  coast  of  southern  New  Brunswick  are  the  Ooldbrook  and 
Coastal  series,  essentially  like  those  of  Newfoundland.  The  Coastal 
group  may  perhaps  be  of  later  age  than  the  Huronian  proper,  although 
pre-Cambrian. 
