The  author  wishes  to  express  his  thanks  to  Dr.  G.  P.  Merrill  of  the
United  States  National  Museum  for  many  helpful  suggestions  received
during  the  course  of  his  work;  and  to  Mr.  S.  Geijsbeek  of  Seattle,  Wash.,
for  assistance  rendered  during  the  compilation  of  the  manuscript.
Acknowledgments  are  similarly  due  to  Professors  C.  S.  Prosser  and
E.  Orton,  Jr.,  of  Ohio  State  University;  Prof.  H.  A.  Wheeler  and  Mr.
Lemon  Parker  of  St.  Louis,  Mo.;  Prof.  I.  A.  Williams,  Iowa  Geological
Survey;  Dr.  E.  A.  Smith,  Alabama  Geological  Survey;  Dr.  G.  P.  Grims-
ley,  West  Virginia  Geological  Survey;  Dr.  W.  B.  Clark,  Maryland  Geo-
logical Survey;  Dr.  D.  H.  Newland,  New  York  Geological  Survey;  Dr.
iv  PREFACE

H.  B.  Kummel,  New  Jersey  Geological  Survey;  and  Mr.  H.  Leigh  ton,
Cornell  University.  Dr.  W.  B.  Phillips,  former  State  Geologist  of  Texas,
has  kindly  given  the  author  permission  to  publish  the  facts  relating  to
that  State.
For  the  loan  of  cuts  the  writer  is  indebted  to  the  American  Clay
Machinery  Co.,  Bucyrus,  Ohio;  Chambers  Brothers  Co.,  Philadelphia,
Pa.;  Henry  Martin  Machine  Co.,  Lancaster,  Pa.;  and  Bergstrom  &  Bass,
Brooklyn,  N.  Y.  Many  others  have  kindly  loaned  photographs,  and  to
each  of  these  acknowledgment  is  made  under  the  respective  illustrations.
CORNELL  UNIVERSITY,  ITHACA,  N.  Y.,
July,  1906.
