560  PRE-CAMRRIAN    GEOLOGY   OF    NORTH   AMERICA. 
superposition,  as  the  strata  are  involved  in  great  confusion.  The 
marbles  of  the  State  are  divided  into  three  systems — Primary,  Ta- 
conic,  and  New  York.  There  are  few  Primary  marbles  in  the  State. 
The  Taconic  marbles  occur  throughout  the  range  of  limestone  from 
Massachusetts  to  the  northern  part  of  Madison  County.  The  Taconic 
system  includes  roofing  slate,  Taconic  slate,  Sparry  limestone,  mag- 
nesian  slates,  Stockbridge  limestone,  and  granular  quartz  rock. 
Adams,20  in  1846,  includes  the  Taconic  and  Primary  systems  of  his 
previous  reports  under  the  general  term  Azoic  stratified  rocks,  as 
simply  expressing  the  fact  of  the  absence  of  organic  remains,  for  it 
is  not  to  be  assumed  that  the  Azoic  Taconic  rocks  are  more  ancient 
than  the  Paleozoic,  and  the  same  remarks  may  be  said  of  the  so- 
called  Primary  rocks,  not  a  small  portion  of  which  may  be  as  meta- 
morphic  and  recent  as  the  Champlain  rocks.  In  the  Green  Moun- 
tains are  quartz  rock,  gneiss,  talciferous  limestone,  quartz  gneiss,  and 
limestone,  which  are  supposed  to  be  Taconic.  Dikes  of  greenstone 
cut  through  all  the  divisions  of  the  stratified  rocks  and  are  therefore 
more  recent  than  any  of  them. 
Adams,21  in  1847,  gives  additional  details  as  to  the  rock  occurrences 
in  particular  localities. 
Thompson,22  in  1856,  states  that  the  Green  Mountains  form  the 
center  of  an  anticlinal  axis,  the  dips  increasing  both  east  and  west 
from  the  principal  summits.  Slates,  schists,  and  quartzites  are  found, 
which  contain  a  few  obscure  fossils  and  are  referred  to  the  Taconic 
system. 
Hitchcock  (Edward),23  in  1861,  divides  the  rocks  in  general  into 
stratified  or  aqueous  and  unstratified  or  igneous.  With  the  latter 
are  placed  granitic,  trappean,  and  volcanic  rocks.  The  former  are 
subdivided  into  fossiliferous  and  unfossiliferous,  or  Azoic.  In  this 
latter  division  are  placed  clay  slate,  quartz  rock,  mica,  schist,  talcose 
schist,  and  chlorite  schist,  steatite  or  soapstone,  serpentine,  hornblende 
schist,  gneiss,  and  crystalline  limestone.  The  important  practical 
question  with  respect  to  the  metamorphic  rocks  is,  What  was  the 
original  rock  from  which  the  metamorphosed  deposit  was  derived? 
In  not  a  few  instances  so  complete  has  been  the  metamorphism  that 
it  can  not  be  told  whether  the  rock  belongs  to  the  oldest  of  the  crystal- 
line rocks  or  is  earlier  than  the  Silurian  or  Cambrian.  While  the 
degree  of  metamorphism  gives  no  clue  as  to  the  age  of  the  rocks,  from 
other  evidence  it  is  probable  that  most  of  the  highly  metamorphosed 
rocks  of  Vermont  are  altered  Devonian  and  Silurian.  In  the  west- 
ern part  of  the  State,  and  especially  that  part  of  New  York  that  lies 
southwesterly,  are  found  these  fossiliferous  rocks  but  little  altered, 
and  these  form  a  starting  point  for  the  Green  Mountain  rocks  and 
those  farther  east. 
