562  PRE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
The  pre-Potsdam  rocks  of  Vermont  are  called  Laurentian  or  Hypo- 
zoic,  although  it  does  not  follow  that  they  are  not  equivalent  with 
the  fossiliferous  series  elsewhere.  It  is  only  believed  that  if  fossils 
once  existed  in  them  they  have  been  obliterated.  Of  the  Hypozoic 
rocks,  Vermont  contains,  so  far  as  known,  only  a  small  belt,  which  is 
the  eastern  edge  of  an  immense  development  of  the  same  in  New 
York.  These  are  referred  here  because  there  seems  to  be  a  discord- 
ance in  the  stratification  between  these  rocks  and  the  Lower  Silurian, 
to  which  they  are  adjacent.  The  oldest  of  the  Paleozoic  series  lies 
directly  upon  the  Hypozoic,  at  least  at  one  point.  As  we  approach 
the  Green  Mountains,  metamorphism  has  so  nearly  destroyed  the 
fossils  that  the  identification  of  the  strata  becomes  extremely  prob- 
lematical, until  at  length  the  clue  is  lost  entirely  and  the  age  of  the 
formations  can  only  be  conjectured;  hence  they  are  distinguished 
mainly  by  lithological  criteria  and  are  grouped  into  a  third  class, 
Azoic  rocks.  Probably  most  of  the  Azoic  rocks  of  Vermont  will  be 
found  to  be  more  recent  than  the  Laurentian  of  Logan.  The  fossil- 
iferous rocks  are  sometimes  found  under  those  that  are  more  crystal- 
line and  nonfossiliferous,  and  these  cases  are  thought  to  be  the  result 
of  inversions.  Certain  great  thicknesses  of  schists  of  uniform  char- 
acter are  regarded  as  folded  several  times  so  as  to  be  vertical,  because 
otherwise  the  thickness  of  the  series  would  be  enormous.  The  talcose 
conglomerates,  talcoid  schists,  Georgia  group,  etc.,  are  referred  to  the 
Potsdam  and  later  formations.  The  Georgia  group  and  sand  rock 
may  possibly  be  Primordial.  The  talcose  conglomerate  is  placed  as 
a  continuation  of  the  Quebec  group  and  Sillery  sandstones  of  Can- 
ada. 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,  Stockbridge  limestone,  and  gran- 
ular 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  that  the  cases  of  subsequent  thorough  altera- 
tion were  exceptional.  The  theory  of  the  Laurentian  age  of  the 
Azoic  rocks  of  New  England  and  the  theory  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  Lau- 
rentian, because  it  underlies  the  Silurian,  because  it  is  immensely 
thicker,  and  because  the  fossils  which  it  contains  are  different  from 
those  of  the  Silurian. 
Hitchcock  (C.  H.),24  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 
