358  PRE-CAMBRIAN    ROCKS    (>F    NORTH    AMERICA. 
Btj.i..  86. 
wise  the  thickness  of  the  series  would  be  enormous.  The  talcose  con 
glomerates,  talcoid  schists,  Georgia  group,  etc.,  are  referred  to  the 
Potsdam  and  later  formations.  The  Georgia  group  and  sand-rock  may 
be  possibly  Primordial.  The  talcose  conglomerate  is  placed  as  a  con- 
tinuation of  the  Quebec  group  and  Sillery  sandstones  of  Canada.  The 
Taconic  system  is  regarded  as  having  an  extension  into  Vermont,  and 
to  it  are  referred  the  black  Taconian  roofing  slates,  Sparry  limestone, 
magnesian  slate,  Stoekbridge  limestone  and  granular  quartz  rock,  with 
associated  talcose  beds,  the  thickness  of  the  whole  being  25,200  feet. 
It  is  supposed  that  the  chief  action  of  metamorphism  was  in  the 
Laurentian  system  and  the  cases  of  subsequent  thorough  alteration  ex- 
ceptional. The  theory  of  the  Laurentian  age  of  the  Azoic  rocks  of  New 
England  and  that  of  the  Cambrian  age  of  the  Taconic  system  stand  or 
fall  together.  The  Taconic  system  is  regarded  as  older  than  the  Lower 
Silurian  and  newer  than  the  Laurentian,  because  it  underlies  the  Silu- 
rian, because  it  is  immensely  thicker,  and  because  the  fossils  which  it 
contains  are  different  from  those  of  the  Silurian. 
Hitchcock,  (Ohas.  H.),22  in  1861,  gives  a  lithological  treatment  of 
the  Azoic  rocks  of  Vermont.  They  are  divided  into  the  following  groups 
which  are  described  in  detail:  Gneiss,  hornblende-schist,  mica  schist, 
clay-slate,  quartz-rock,  talcose  schist,  serpentine  and  steatite,  and  sac- 
charoid  limestone.  Cleavage  has  a  widespread  occurrence,  although  it 
is  believed  that  the  strike  and  dip  of  lamination  and  stratification  gen- 
erally correspond.  The  granite- gneiss  approaches  so  near  to  granite 
that  in  hand  specimens  the  two  rocks  can  not  be  distinguished  from 
each  other.  The  granite  of  the  two  Aseutneys  seems  to  have  cut  across 
the  strata  of  the  calciferous  mica-schist  quite  a  distance  into  the  gneiss, 
although  the  granite  is  regarded  as  of  metamorphic  origin,  (lay-slate 
often  passes  by  insensible  gradations  into  mica-schist,  which  is  re- 
garded as  a  modified  fragmental  rock.  Associated  with  the  talcose 
schists  and  constituting  an  integral  part  of  the  formation  are  clay-slate, 
gneiss,  quartz-rock,  sandstones,  and  conglomerates.  Igneous  rocks, 
both  trap  and  granite,  are  also  associated  with  this  formation. 
Hitchcock,  (Edward),2*  in  1861,  considers  in  detail  the  relations  of 
the  granitic  to  the  other  rocks  and  considers  the  origin  of  the  granite. 
This  rock  is  found  interstratified  with  slate,  limestones  and  mica-schists 
in  many  localities.  At  the  " Narrows,"  in  the  northern  part  of  Coven- 
try, the  number  of  alternations  is  very  large,  the  thickness  of  the  differ- 
ent layers  of  granite  varying  from  1  to  7  feet. 
The  conglomeratic  syenites  of  Whately,  Massachusetts,  and  Aseutney, 
Granby  and  Barnet,  Vermont,  which  contain  fragments,  are  described 
in  detail.  These  may  be  described  as  conglomerates  whose  cement  is 
syenite,  or  a  syenite  through  which  is  scattered  pebbles  mechanically 
rounded.  Generally  the  pebbles  are  more  or  less  metamorphosed  and 
sometimes  almost  converted  into  syenite  subsequent  to  their  introduc- 
tion.   In  their  present  state  white  quartz,  mica-schist,  hornblende-schist. 
