III!  PRE-CAMBRIA1*    ROCKS    OF    NuKTH    AMERICA.    .        [bull.8«. 
The  l  ruth  of  this  is  well  illustrated  by  comparing  the  eastern  and  west 
era  regions  of  the  United  States.  In  the  former  powerful  dynamic 
movements  have  occurred  until  late  in  Paleozoic  time,  as  a  result  of 
which  the  Cambrian,  Silurian,  and  Devonian  rocks  over  large  areas 
have  not  been  separated  from  the  pre  Cambrian.  In  certain  areas  tliis 
separation  has  been  accomplished,  but  only  by  the  most  accurate  and 
painstaking  application  of  modern  methods.  In  parts  of  Massachu- 
setts and  Vermont  the  areas  covered  and  the  results  reached  have  in- 
volved an  enormous  amount  of  Labor,  although  of  late  paleontology 
has  been  an  important  help  in  unraveling  the  problem.  Under  these 
circumstances   how  much    more  difficult  would  one  expect    it  to  be   to 
separate  the  pre  Cambrian  clastic  series  from  the  Archean! 
Iu  parts  of  the  West,  where  no  close  folding  has  occurred  since  Cam- 
brian time,  it  is  easy  to  separate  the  Cambrian  from  thepre  Cambrian, 
and  in  regions  in  which  metamorphosing  influences  have  not  been  at 
work  tor  a  still  longer  time  it  is  easy  to  separate  the  pre  Cambrian 
elastics  from  the  Archeau.  But  in  other  regions  this  separation  is 
made  with  the  greatest  difficulty,  and  doubtless  over  large  areas  this 
will  never  be  satisfactorily  done.    Just  as  in  the  Appalachians,  in  parts 
of  which  it  may  be  impracticable  to  separate  the  Cambrian  rocks  from 
the  pre  Cambrian  clastic  scries,  if  sncli  exists,  so  it  will  be  tor  a  long 
time  impossible  to  decide  in  some  regions  Upon  sharp  boundary  lines 
between  the  lire  -Cambrian  elastics  and  the  Archean.  Givingfull  force 
to  this  position,  it  is  no  reason  why  the  discrimination  should  not  be 
made  where  it  can  be. 
Recent  work  in  petrography  has  demonstrated  that  'dynamic  move* 
incuts  and  environment,  not  time,  are  the  important  elements  in  the 
obliteration  otclastic  characteristics.  Dynamic  movements  also  destroy 
the  evidences  of  discordances  between  series  where  there  have  been  real 
unconformities.  This  destruction  of  the  evidence  of  structural  breaks 
comes  about  largely  as  the  result  of  an  approaching  parallelism  of  bed- 
ding, caused  by  the  close  (biding,  but  tar  more  important  than  this  is 
the  production  of  a  common  cleavage  and  foliation  with  the  simulta- 
neous development  of  crystalline  schists  from  the  newer  series.  As  a 
consequence  basal  conglomerates  are  often  almost  the  only  means  of 
discriminating  between  the  newer  and  the  older  series,  and  if  the  meta- 
morphosing Influences  arc  powerful  enough  to  destroy  the  pebbled  char 
actcrofsuch  beds,  changing  them  into  schists  or  gneisses,  as  has  occurred 
in  many  places  in  the  Cambrian  of  the  Appalachians,  this  means  of 
detecting  a  break  between  series  is  also  lost.  The  problem  is  rendered 
still  more  difficult  because  of  the  fact  that  often  when  there  is  a  real 
unconformity  there  has  been  originally  no  basal  conglomerate.  At 
many  localities  in  the  far  West  thebasemenl  fossil iferous  scries  are  built 
up  o\'  tin4,  constituent  minerals  of  the  underlying  rocks  rather  than  of 
large  fragments  of  them,  and  even  when  not  folded  have  sometimes  so 
closely  simulated  the  original  rocks  that  geologists  have  been  at  a  loss 
