﻿34 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  bed.s 
  northward 
  is 
  due 
  to 
  a 
  thinning 
  out 
  of 
  the 
  exposed 
  portion 
  of 
  

   the 
  strata, 
  as 
  can 
  be 
  readily 
  seen 
  in 
  the 
  Lockport 
  Hmestone 
  bed, 
  

   which 
  is 
  less 
  than 
  30 
  feet 
  thick 
  at 
  Lewiston, 
  but 
  more 
  than 
  80 
  feet 
  

   at 
  the 
  falls, 
  increasing 
  in 
  thickness 
  southward 
  to 
  250 
  feet 
  or 
  more. 
  

   Where, 
  however, 
  the 
  strata 
  are 
  not 
  exposed 
  on 
  the 
  surface, 
  i. 
  e. 
  

   where 
  they 
  are 
  only 
  shown 
  in 
  sections 
  under 
  cover 
  of 
  the 
  overlying 
  

   rock, 
  no 
  such 
  thinning 
  is 
  seen. 
  This 
  may 
  be 
  observed 
  in 
  the 
  case 
  

   of 
  the 
  Clinton 
  beds 
  and 
  the 
  upper 
  Medina 
  sandstones. 
  In 
  some 
  

   cases 
  these 
  beds 
  are 
  seen 
  to 
  even 
  thin 
  southward, 
  as 
  proved 
  by 
  bor- 
  

   ings. 
  The 
  thinning 
  of 
  these 
  strata 
  does 
  not, 
  as 
  is 
  often 
  assumed, 
  

   mark 
  the 
  original 
  thinning 
  of 
  the 
  beds 
  toward 
  the 
  shore 
  on 
  the 
  

   north, 
  but 
  is 
  evidently 
  due 
  to 
  erosion. 
  A 
  brief 
  resume 
  of 
  the 
  origin 
  

   of- 
  the 
  various 
  strata 
  will 
  make 
  this 
  clear. 
  

  

  The 
  Medina 
  sandstone 
  is 
  an 
  ancient 
  shore 
  and 
  shallow 
  water 
  de- 
  

   posit, 
  as 
  will 
  be 
  more 
  fully 
  pointed 
  out 
  in 
  chapter 
  3. 
  The 
  

   sands 
  and 
  gravels, 
  which 
  with 
  some 
  finer 
  muds, 
  make 
  up 
  this 
  

   rock, 
  are 
  all 
  derived 
  from 
  some 
  preexisting 
  land. 
  The 
  only 
  

   source 
  of 
  supply 
  was 
  the 
  old 
  Laurentian 
  land 
  on 
  the 
  north 
  

   and 
  the 
  Appalachian 
  old-land 
  on 
  the 
  south. 
  It 
  is 
  true 
  that, 
  

   owing 
  to 
  the 
  elevation 
  at 
  the 
  beginning 
  of 
  Siluric 
  time, 
  some 
  

   of 
  the 
  pre-Siluric 
  stratified 
  rocks 
  may 
  have 
  been 
  raised 
  above 
  the 
  

   sealevel 
  and 
  added 
  to 
  the 
  old-land, 
  and 
  that 
  part 
  of 
  the 
  Medina 
  

   sands 
  may 
  have 
  been 
  derived 
  from 
  these. 
  Even 
  then 
  the 
  largest 
  

   amount 
  of 
  detritus 
  was 
  probably 
  derived 
  from 
  the 
  crystalline 
  old- 
  

   lands, 
  the 
  progressive 
  accumulation 
  of 
  1200 
  feet 
  of 
  Medina 
  rock 
  

   marking 
  a 
  corresponding 
  subsidence 
  and 
  a 
  concomitant 
  encroach- 
  

   ment 
  of 
  the 
  seashore 
  of 
  the 
  Medina 
  sea 
  on 
  the 
  old-land. 
  Thus 
  the 
  

   Medina 
  deposits 
  gradually 
  overlapped 
  the 
  Ordovicic 
  and 
  Cam- 
  

   bric 
  deposits 
  and 
  probably 
  eventually 
  came 
  to 
  rest 
  entirely 
  on 
  the 
  

   crystalline 
  pre-Cambric 
  rocks. 
  Continued 
  subsidence, 
  at 
  least 
  in 
  

   the 
  Niagara 
  region, 
  produced 
  the 
  purification 
  of 
  the 
  water, 
  so 
  that 
  

   eventually 
  the 
  limestones 
  of 
  the 
  Clinton 
  epoch 
  could 
  be 
  formed 
  in 
  a 
  

   region 
  remote 
  from 
  that 
  in 
  which 
  terrigenous 
  material 
  was 
  ac- 
  

   cumulating. 
  This 
  was 
  likewise 
  true 
  of 
  the 
  Lockport 
  limestone, 
  

   which 
  was 
  deposited 
  after 
  an 
  interval, 
  during 
  which 
  the 
  calcareous 
  

   shales 
  separating 
  the 
  two 
  limestone 
  series 
  accumulated. 
  While 
  

  

  