524  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
impossible  to  work  out  the  structure  of  crystalline  rock  series  which 
have  a  true  detrital  succession,  but  it  is  better  for  the  present  and  the 
future  that  no  structure  be  presented  than  a  false  one. 
After  working  out  .the  structure  of  one  district  in  a  geological  basin, 
adjacent  districts  may  be  mapped  much  more  rapidly.  Under  proper 
checks  the  lithological  character  of  individual  beds  may  be  assumed  to 
remain  the  same.  Sete  of  like  formation  occurring  in  the  same  order 
may  be  assumed  to  be  the  same  group  of  formations.  And  perhaps 
most  useful  of  all  are  discordant  relations  between  series.  In  correla- 
tions from  region  to  region,  without  the  assistance  of  paleontology  it 
will  probably  not  be  possible  to  carry  the  analogy  further  than  series; 
and  in  far  distant  regions  even  the  general  lithological  likeness  and 
similarity  of  position  of  series  is  not  sufficient  warrant  for  placing  them 
opposite  each  other  in  the  time  column. 
If  the  foregoing  principles  are  true,  it  is  plain  that  in  working  out  the 
structure  of  a  new  region  local  names  should  be  applied  to  the  forma- 
tions and  series.  When  the  time  comes  that  fuller  knowledge  enables 
them  to  be  safely  correlated  with  the  series  to  which  classical  names 
have  been  applied,  this  may  be  done,  and  the  local  names  will  not  be  less 
serviceable  to  designate  particular  parts  of  these  general  series. 
The  area  in  North  America  in  which  detailed  mapping  has  been  done 
with  a  resultant  proper  understanding  of  the  structural  relations  of  the 
pre-Oambrian  is  surprisingly  small.  Scarcely  a  crystalline  area  on  the 
continent  has  escaped  the  rapid  geologist  who  has  passed  over  a  region 
and  upon  a  few  facts  of  uncertain  value  publishes  structural  conclu- 
sions which  are  not  to  be  verified  by  future  work.  The  districts  care- 
fully studied  include  the  original  Huronian  of  lake  Huron,  several 
small  areas  about  Ottawa  and  between  the  Ottawa  river  and  lake  On- 
tario, a  few  small  areas  about  the  lake  Superior  region,  a  small  part  of 
western  Massachusetts  and  a  part  of  Maryland.  Even  in  these  districts 
the  work  at  many  points  is  rather  old  and  to  a  certain  extent  unsatis- 
factory. Before  reliable  maps  can  be  obtained  this  old  work  must  be 
thoroughly  revised  in  the  light  of  the  recent  advances  in  the  methods 
of  study  of  the  crystalline  rocks.  By  this  it  is  not  implied  that  the 
more  general  work  done  is  not  of  superlative  value  and  of  necessity 
must  precede  the  more  accurate  accounts.  A  beginning  has  been  made 
in  American  pre-Cambrian  stratigraphy,  but  the  great  mass  of  work 
remains  for  the  future. 
RESULTS  IN  AMERICA  AND  EUROPE   COMPARED. 
This  volume  has  already  become  too  long  to  attempt  to  make  a  de- 
tailed comparison  between  America  and  Europe  as  to  the  results  reached 
in  pre-Oambrian  stratigraphy.  Also,  I  am  wholly  unfamiliar  with  Eu- 
ropean ground  and  am  but  imperfectly  acquainted  with  the  literature; 
hence  I  would  not  be  warranted  in  making  the  attempt,  even  if  space 
permitted.    It  may,  however,  be  well,  without  giving  any  detailed  facts 
