382  PKE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
SUMMARY   OF   RESULTS. 
In  this  discussion  no  attempt  will  be  made  to  consider  in  detail  the 
structures  worked  out  by  the  various  authors  in  individual  districts, 
but  rather  to  indicate  the  general  character  of  the  results  which  have 
been  reached. 
It  is  plain  that  the  knowledge  of  the  pre  Cambrian  structure  of  the 
New  England  states  is  in  a  very  unsatisfactory  condition.  Even  in 
such  a  district  as  that  of  eastern  Massachusetts,  including  the  Boston 
basin,  there  is  the  greatest  diversity  of  opinion,  a  district  which  one 
would  naturally  expect  would  have  been  systematically  studied  and 
certain  results  reached.  In  western  Massachusetts,  where  the  most 
elaborate  of  the  older  surveys  has  been  made,  the  rock  successions 
given  from  time  to  time  and  the  correlations  with  other  series  have  varied 
greatly.  These  areas  are  particularly  mentioned  because  most  studied. 
What  is  here  true  is  still  more  emphatically  the  case  with  the  remainder 
of  the  region. 
While,  then,  systematic  work  in  the  New  England  states  is  but  fairly 
begun,  one  great  structural  fact  is  clearly  apparent,  that  large  areas  of 
rocks  which  have  commonly  been  regarded  as  pre-Cambrian  are  prov- 
ing to  be  Cambrian  or  post- Cambrian.  This  is  most  marked  in  west- 
ern Massachusetts,  where  the  recent  work  of  Dana,  Pumpelly,  Dale, 
Wolff,  Emerson  and  Walcott  has  established  the  conclusion  that  the 
pre-Cambrian  rocks  are  confined  to  small  areas.  The  results  reached 
in  this  district  show  that  it  will  not  do  to  call  the  crystalline  rocks 
pre-Cambrian  and  the  uncrystalline  ones  post-Cambrian,  a  principle 
clearly  recognized  by  Adams  and  the  elder  Hitchcock  many  years  ago. 
Over  large  areas  mica-slates,  mica-schists  and  evenly  granular  gneisses, 
have  been  demonstrated  beyond  all  question  to  belong  to  the  Cambrian 
or  post-Cambrian.  If  in  the  remainder  of  New  England  the  pre-Cam- 
brian rocks  shrink  in  proportion  as  they  have  done  in  the  districts 
closely  studied,  they  will  not  occupy  more  than  a  fraction  of  the  area 
usually  assigned  to  them. 
If  it  be  true  that  a  very  large  proportion  of  the  New  England  rocks 
which  have  been  supposed  to  be  pre-Cambrian  are  really  Cambrian  or 
post-Cambrian,  it  is  evident  that  the  reference  of  them  to  the  Montal- 
ban,  Norian,  Huronian,  Laurentian,  etc.,  is  wholly  unwarranted.  In 
the  districts  already  referred  to  where  definite  knowledge  is  available 
many  of  the  rocks  before  referred  to  these  various  series  are  being 
divided  into  two  classes,  sedimentary  and  igneous,  and  are  being  dis- 
tributed from  the  Cambrian  to  the  Carboniferous. 
As  yet  it  can  not  be  positively  said  of  any  district  that  the  prd 
Cambrian  rocks  can  be  subdivided  upon  a  structural  basis,  although 
recent  unpublished  results  by  Pumpelly  and  his  assistants  in  the  Green 
mountains  indicate  that  it  may  be  possible  to  do  this  in  the  future. 
This  New  England  region  is  one  of  great  interest  as  being  the  first 
iu  which  it  was  clearly  shown  by  structural  work  that  fragmental  rocks 
