582  PKE-CAMBEIAN    GEOLOGY   OF   NORTH   AMERICA. 
Mather,73  in  1834,  divides  the  rocks  of  Connecticut  into  gneiss, 
hornblende  slate,  mica  slate,  granular  feldspar,  granular  quartz,  syen- 
ite, granite,  and  limestone.  ,  The  strata  generally  show  themselves 
in  long  belts  extending  unbroken  to  a  considerable  distance.  The 
thickness  of  the  gneiss  at  one  locality  is  not  less  than  10,000  feet.  At 
Lebanon  the  gneiss  surrounding  a  great  part  of  the  syenite  dips  so  as 
apparently  to  pass  under  the  latter  rock.  The  syenite  is  not  strati- 
fied, but  the  granite  is  partly  stratified.  The  limestone  occurs  in  beds 
1  to  20  feet  thick,  embraced  in  the  contorted  gneiss  in  the  north- 
western part  of  Stonington. 
Percival,74  in  1842,  divides  the  consolidated  rocks  of  Connecticut 
into  Primary,  Secondary,  and  Trap  rocks.  The  Primary  rocks  oc- 
cupy the  greater  portion  of  the  surface  of  the  State  and  are  divided 
into  western  and  eastern  sections.  These  Primary  rocks  are  formed 
entirely  of  original  materials,  exhibiting  no  appearance  of  any  frag- 
ment or  remains  of  any  anterior  formation.  The  Trap  rocks  are 
chiefly  connected  with  the  Secondary  rocks,  although  they  also  cut 
the  Primary.  These  are  regarded  as  intrusive  and  igneous.  In  de- 
scribing the  Primary  rocks  the  term  parallel  is  preferred  to  strati- 
fied, as  simply  expressing  the  arrangement  of  the  minerals  without 
implying  any  opinion  as  to  the  mode  of  formation.  The  western 
Primary  system,  which  extends  on  the  west  and  southwest  into  the 
State  of  New  York  and  on  the  north  into  Massachusetts,  is  divided 
into  a  large  number  of  local  formations,  including  the  classes  mica 
slate,  argillite,  granite,  calciferous  schist,  and  limestone.  The  forma- 
tions as  a  whole  present  a  series  of  parallel  ridges  which  have  a  gen- 
eral curvature  with  convexity  toward  the  east.  The  central  portion 
exhibits  a  series  of  granitic  and  micaceous  alternations,  which  appear 
partly  as  elongated  bands  and  partly  as  isolated  nuclei,  generally 
granitic,  around  which  the  more  micaceous  formations  are  concen- 
trically arranged.  The  eastern  Primary  system  is  divided  into  sev- 
eral main  groups,  the  rocks  here  including  gneiss  and  micaceous  and 
chloritic  rocks.  In  one  of  the  granitic  formations  the  arrangement 
consists  of  a  central  nucleus  of  granite  gneiss  surrounded  by  narrow 
concentric  rings  of  various  characters.  The  zone  immediately  ad- 
joining the  granitic  nucleus  is  characterized  by  the  almost  constant 
presence  of  anthophyllite.  From  the  coarsest  granites  to  the  finest 
and  most  uniform  schist  the  structure  is  characterized  by  a  parallel 
arrangement  of  the  mineral  constituents.  Continual  alternations  of 
series  of  granitic,  micaceous,  hornblendic,  and  various  other  forma- 
tions were  observed.  The  Trap  rocks  occurring  in  the  Primary  and 
Secondary  formations  are  so  nearly  alike  that  they  can  not  be  sepa- 
rated, but  in  the  Primary  rocks  there  is  no  conformity  between  the 
Traps  and  the  adjacent  formations,  such  as  prevails  in  the  Secondary. 
