SUMMARY    OF    GENERAL  LITERATURE.  83 
strings  and  masses  composed  of  iron  ore,  bisilicates,  and  mica,  as  well 
as  larger  porphyritic  crystals  of  plagioclase.  Even  when  tolerably 
constant  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 
interstratified  with  the  Laurentian  gneiss,  and  in  one  of  them  to 
merge  imperceptibly  into  it.  The  original  relations  of  the  rocks  are, 
of  course,  much  obscured  by  the  effects  of  subsequent  heat  and  pres- 
sure. The  evidence  at  present,  however,  seems  to  indicate  that  the 
anorthosites  are  the  result  of  some  kind  of  extravasation  which  in 
early  times  corresponded  to  what  in  modern  times  we  call  volcanic 
eruption. 
Daavson  (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 
of  orthoclase  gneiss  associated  with  crystalline  schists,  and  locally 
quartzites  and  limestone. 
No  one  who  has  studied  the  typical  districts  of  Ottawa  Eiver  can 
doubt  for  a  moment  that  they  are  regularly  bedded  deposits,  and  that 
in  the  Middle  Laurentian  those  conditions  which  in  later  periods  have 
produced  beds  of  limestone,  sandstone,  iron  ore,  and  even  of  coal,  were 
already  in  operation  on  a  gigantic  scale.  At  the  same  time  it  may  be 
admitted  that  some  areas  of  the  lower  gneiss  may  be  cooled  portions 
of  an  original  igneous  mass,  and  that  many  of  the  schistose  rocks  may 
be  really  bedded  igneous  materials. 
Laurentian  rocks  compose  the  nucleus  of  the  island  of  Newfound- 
land and  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  dis- 
turbed, 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,  lime- 
stone, and  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  Wesi 
overlie  these  lower  slates. 
On  the  coast  of  southern  New  Brunswick  are  the  Coldbrook  and 
Coastal  series,  essentially  like  those  of  Newfoundland.     The  Coastal 
