﻿FIFTH 
  REPORT 
  OF 
  THE 
  DIRECTOR 
  I908 
  

  

  169 
  

  

  portance 
  for 
  the 
  understanding 
  of 
  the 
  geological 
  structure 
  of 
  the 
  

   eastern 
  United 
  States. 
  

  

  As 
  a 
  third 
  group 
  of 
  depositional 
  inliers 
  may 
  be 
  distinguished 
  the 
  

   relatively 
  rare 
  and 
  unimportant 
  case 
  where 
  current 
  mounds 
  and 
  

   ridges 
  have 
  been 
  buried 
  under 
  younger 
  sediments 
  and 
  later 
  become 
  

   exposed 
  as 
  inliers. 
  An 
  example 
  of 
  this 
  kind 
  is 
  furnished 
  by 
  the 
  

   Le 
  Claire 
  lim'festone 
  of 
  lowa^ 
  that 
  forms 
  mounds 
  50 
  feet 
  high 
  or 
  

   over 
  and 
  in 
  regard 
  to 
  which 
  it 
  was 
  already 
  suggested 
  by 
  Hall 
  that 
  

   " 
  at 
  the 
  close 
  of 
  the 
  Niagara 
  huge 
  mounds 
  and 
  ridges 
  were 
  built 
  

   on 
  the 
  bottom 
  of 
  the 
  shallow 
  Silurian 
  sea, 
  in 
  part 
  by 
  the 
  accumu- 
  

   lation 
  ill 
  situ 
  of 
  corals, 
  crinoids 
  and 
  molluscous 
  shells, 
  and 
  in 
  part 
  

   by 
  the 
  drift 
  of 
  calcareous 
  sediments 
  under 
  strong 
  currents." 
  

  

  Fig. 
  6 
  Lens 
  or 
  reef 
  resting 
  upon 
  Clinton 
  limestone 
  and 
  extending 
  into 
  Rochester 
  shale. 
  

   (Copy 
  from 
  Sarle) 
  

   Fig. 
  7 
  Same 
  worn 
  down 
  and 
  forming 
  inlier 
  

  

  As 
  these 
  current 
  mounds 
  later 
  project 
  from 
  the 
  softer 
  and 
  more 
  

   easily 
  eroded 
  rocks 
  that 
  once 
  buried 
  them, 
  and 
  thus 
  form 
  inliers, 
  

   so 
  also 
  limestone 
  mounds 
  which 
  grow 
  on 
  the 
  bottom 
  of 
  the 
  ocean 
  

   througih 
  the 
  action 
  of 
  corals, 
  bryozoans 
  and 
  the 
  accumulation 
  of 
  

   mollusks, 
  may 
  become 
  buried 
  in 
  shales 
  and 
  later 
  project 
  from 
  these 
  

   as 
  " 
  lenses." 
  Such 
  reef 
  structures 
  have 
  been 
  described 
  by 
  Sarle- 
  

   as 
  reefs 
  in 
  the 
  Clinton 
  formation 
  of 
  western 
  New 
  York. 
  When 
  

   they 
  indicate 
  an 
  older 
  horizon 
  than 
  the 
  overlying 
  shale, 
  they 
  con- 
  

   stitute 
  true 
  inliers. 
  This 
  is 
  frequently 
  the 
  case 
  in 
  western 
  New 
  

   York, 
  where 
  they 
  represent 
  the 
  top 
  of 
  the 
  Clinton 
  limestone 
  and 
  

  

  ]^ 
  Iowa 
  Geol. 
  Siir. 
  Rep't, 
  11:305. 
  

  

  "Sarle, 
  C. 
  J. 
  Amer. 
  Geol. 
  1901. 
  28:282. 
  

  

  