PREFACE. 
In  the  preparation  of  this  book  great  difficulty  has  been  encountered 
because  of  the  unequal  value  of  statements  of  fact  by  different  men. 
Oftentimes  in  regions  with  which  the  writer  is  not  familiar  it  is  impossible 
to  discriminate  surely  between  good  and  poor  work.  In  certain  cases 
in  which  reports  have  read  plausibly,  an  examination  of  the  purported 
facts  in  the  field  with  the  accounts  in  hand  has  shown  the  descriptive 
parts  to  be  so  inaccurate  as  to  render  the  conclusions,  while  apparently 
well  founded,  wholly  valueless.  But  in  general,  when  an  author  of  this 
class  has  written  much,  discrepancies  between  the  statements  of  facts 
appear. 
Those  who  change  their  opinions  are  of  different  classes.  A  man  who 
progresses  must  change  his  views,  but  the  descriptive  parts,  given 
while  an  old  view  was  held,  ought  to  be  still  more  useful  for  support  of 
the  new  position.  The  old  view  may  have  contained  a  large  clement  of 
truth,  and  is  perhaps  included  in  the  newer,  larger  position.  In  reports 
the  part  that  purports  to  be  facts  should  be  wholly  separable  from 
the  general  theories  held,  just  as  detailed  maps,  if  rightly  constructed, 
indicate  the  observations  made  and  the  generalizations  drawn.  More 
frequently  than  not,  facts  and  theories  are  so  inextricably  mingled 
that  no  independent  judgment  can  be  reached  as  to  the  correctness 
of  the  conclusions,  and  often  the  facts  of  a  report  can  not  be  used  even 
by  one  personally  familiar  with  the  districts  of  which  the  report  treats. 
The  conclusions  of  another  class  of  geologists  are  a  series  of  guesses, 
which  generally  serve  no  purpose  except  that  when  any  one  of  the 
numerous  guesses  has  been  established  by  the  patient  work  of  an  inves- 
tigator, the  conclusion  is  at  once  claimed  as  a  prior  discovery  of  the 
guesser.  Sometimes  the  discoveries  announced  by  a  writer  almost  or 
quite  simultaneously,  are  wholly  inconsistent  with  one  another  and 
with  the  facts  which  are  described;  for  as  with  other  men,  so  with 
geologists,  many  opinions  are  held  at  the  same  time  which  logically 
are  exclusive  of  one  another.  Still  another  group  of  writers  early 
reach  a  general  theory  as  to  the  definite  order  of  the  evolution  of  the 
world.  A  person  of  this  group  year  after  year  repeats  the  old  state- 
ments and  conclusions  without  any  reference  to  the  establishment  of 
their  falsity.  More  often  than  not,  he  is  one  who  has  done  little  or 
no  systematic  detailed  field  work  in  any  region.  All  facts  and  conclu- 
sions which  bear  in  his  direction  are  hailed  as  discoveries;  "  all  is  grist 
15 
