232  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bitu,  86. 
rentian  because  older  than  the  Silurian  rocks,  which  contain  a  Primor- 
dial zone,  as  well  as  on  account  of  their  general  lithological  resemblance 
to  the  ancient  rocks  of  the  Laurentian  system.  The  Lauren tiau  is 
separated  from  the  Primordial  beds  by  an  accumulation  of  trappean 
and  tufaceous  strata  which  is  supposed  to  be  of  Huronian  age.  The 
lower  division  of  the  Laurentian  consists  of  diorites,  syenite,  grani- 
toid gneiss,  etc.,  while  its  upper  division  consists  of  crystalline  lime- 
stone with  diorite  at  intervals,  greenish  gray  gneiss,  quartzite,  argillite, 
and  slate  conglomerate.  The  contact  of  the  granite  with  the  schistose 
rocks  is  peculiar.  Contained  in  the  granite  are  long  irregular  blocks 
of  the  schistose  rock.  Their  occurrence  suggests  either  a  softening  of 
the  older  series  through  metainorphism  subsequent  to  the  deposition  of 
the  upper  series,  or  else  the  intrusive  character  of  the  granites.  One 
section  of  gneiss  and  granite  in  the  Lower  Laurentian  has  a  thick- 
ness of  12,600  feet.  The  maximum  thickness  of  limestone  and  quartzite 
of  the  Upper  Laurentian  is  not  more  than  a  thousand  feet. 
In  the  Huronian  series  are  placed  the  Coldbrook,  Coastal,  and  Kings- 
ton groups,  which  together  occupy  a  wide  area.  The  Coldbrook  group 
at  several  points  was  observed  to  rest  upon  the  gneissic  and  granitic 
rocks  referred  to  the  Laurentian  system,  and  in  turn  to  be  conformably 
overlain  by  the  slates  of  the  St.  John  group  containing  Primordial 
fossils.  The  Coldbrook  group  consists  of  diorite,  chlorite- schists,  black 
slates,  micaceous  shales,  argillites,  gneissoid  rocks,  and  other  varieties. 
The  rocks  of  the  Coastal  group  consist  of  felsites  and  conglomerates, 
gray  limestones,  and  gray  clay  slates,  gray  chloritic  grits  and  schists, 
micaceous  slate  and  gray  dolomite,  green  and  red  clay  slate,  and  diorite. 
The  Kingston  group  consists  of  shales,  felsites,  diorites,  and  argillites. 
In  Grand  Manan  are  various  crystalline  rocks  which  are  not  definitely 
referred  to  any  period,  but  are  compared  to  the  Huronian.  The  Devon- 
ian and  Huronian  rocks  are  folded  together,  but  no  evidence  is  seen  of 
unconformity  between  the  two  series.  The  Mascarene  series  in  general 
aspect  resembles  the  Huronian  series  of  St.  John  and  King  counties. 
Intrusive  granites  are  found  at  many  points.  In  general  the  contacts 
of  these  granites  with  the  surrounding  rocks  are  not  found,  and  their 
age  is  thus  unknown. 
The  diorites  and  schists  of  Bloomsbury  mountain,  formerly  supposed 
on  strati  graphical  grounds  to  be  more  recent  than  the  Huronian,  resting 
upon  the  St.  John  group  and  overlain  conformably  by  the  Devonian 
sandstones,  are  now  regarded  as  Huronian  on  lithological  grounds, 
being  probably  brought  up  by  a  fault.  At  Musquash  harbor  and  at 
Katclifte's  mill  stream  green  subcrystalline  schists  rest  conformably 
upon  the  Primordial  strata,  but  these  are  believed  to  be  overturns  and 
the  former  Huronian. 
The  rocks  of  the  Coastal  group  at  several  points  overlie  Upper 
Silurian  or  Devonian  strata.  They  were  formerly  described  as  Devo- 
nian; but  as  Hunt  finds  them  similar  to  the  Huronian  in  lithological 
