16  PREFACE. 
which  comes  to  his  mill ; "  while  every  adverse  fact  or  conclusion  is  ex- 
plained out  of  existence  or  dismissed  as  unworthy  of  consideration. 
By  following  continuously  the  summaries  of  the  writings  of  a  geolo- 
gist who  has  been  long  at  work  in  a  region  it  will  generally  not  be  diffi- 
cult to  get  a  fairly  accurate  idea  of  the  value  of  the  work  done. 
During  the  preparation  of  this  review,  I  have  visited  many  districts 
of  North  America,  but  I  have  not  been  able  to  see  important  districts 
which  I  hoped  to  study  before  the  submission  of  this  volume.  With 
the  United  States  side  of  the  lake  Superior  country  I  am  tolerably 
familiar,  having  for  a  number  of  years  given  nearly  the  fall  field  seasons 
to  work  in  various  parts  of  this  region.  Besides  doing  general  work, 
I  have  mapped  in  detail  certain  districts.  From  time  to  time  various 
parts  of  the  Canadian  lake  Superior  region  have  been  visited.  While 
no  systematic  mapping  work  has  been  done  in  regions  other  than  that 
of  lake  Superior,  reconnaissances  and  occasional  detailed  sections  have 
been  made  in  many.  In  the  far  West  these  include  the  Black  hills  of 
Dakota,  southwestern  Montana,  the  Laramie  and  Medicine  Bow  moun- 
tains of  Wyoming ;  the  Uinta  and  Wasatch  mountains  of  Utah ;  and  the 
Quartzite  mountains  and  Front  range  of  Colorado.  In  the  Mississippi 
valley  the  crystalline  region  of  Missouri  has  been  seen.  In  company 
with  Raphael  Pumpelly,  Bailey  Willis,  J.  A.  Holmes,  C.  D.  Walcott,  G.  H. 
Williams,  R.  W.  Ells,  or  alone,  more  or  less  extended  trips  have  been 
made  in  Georgia,  east  Tennessee,  North  Carolina,  eastern  Maryland,  east- 
ern Pennsylvania,  northern  New  Jersey,  southern  New  York,  the  Berk- 
shire hills,  Green  mountains,  Adirondacks,  Hastings  district  of  Ontario, 
and  the  area  of  the  Grenville  series  constituting  the  Original  Laurentian. 
For  the  most  part  this  work  has  not  been  of  such  a  detailed  nature  as 
to  add  greatly  to  previous  knowledge  of  these  regions.  The  aim  has 
been  rather  to  get  such  a  familiarity  with  them  as  would  enable  the 
writer  to  judge  accurately  of  the  results  already  reached.  This  state- 
ment does  not  apply  to  a  part  of  the  Adirondacks,  where  a  somewhat 
closer  study  was  made;  also,  in  North  Carolina  a  nearly  complete  sec- 
tion was  made  from  the  eastern  side  of  the  Piedmont  plain  through  the 
Blue  ridge  to  the  uncrystalline  rocks  of  east  Tennessee.  The  Original 
Huronian  area  has  been  seen  at  three  different  times.  The  first  time, 
in  company  with  the  late  Dr.  R.  D.  Irving,  the  North  channel  was 
coasted  in  a  small  boat  and  the  interior  visited,  so  that  all  of  Logan's 
members  of  this  series  were  seen,  as  well  as  the  underlying  Laurentian. 
In  a  second  trip,  also  in  company  with  Prof.  Irving,  the  Canadian  Pa- 
cific railroad  was  traversed  twice  on  a  hand  car  from  Algoma  Mills  to 
Sudbury,  a  distance  of  100  miles.  The  third  trip,  with  Prof.  Raphael 
Pumpelly,  was  again  along  the  North  channel  of  lake  Huron,  the  object 
of  special  study  being  the  possible  existence  of  two  series  within  the 
Huronian,  and  the  structural  relations  between  the  lowest  Huronian 
and  the  Laurentian. 
