﻿W. 
  TJpham 
  — 
  Epochs 
  and 
  Stages 
  of 
  the 
  Glacial 
  Period. 
  305 
  

  

  ena 
  that 
  have 
  appeared 
  in 
  the 
  photographs, 
  of 
  the 
  primary 
  

   arid 
  secondary 
  sparks. 
  

  

  If 
  it 
  appears, 
  as 
  theory 
  seems 
  to 
  indicate, 
  that 
  electric 
  waves 
  

   travel 
  in 
  air 
  with 
  the 
  velocity 
  of 
  light, 
  it 
  may 
  be 
  that 
  the 
  latter 
  

   can 
  be 
  determined 
  more 
  accurately 
  by 
  an 
  electrical 
  and 
  photo- 
  

   graphic 
  method 
  than 
  by 
  the 
  eye 
  methods 
  which 
  have 
  hitherto 
  

   been 
  used. 
  

  

  Jefferson 
  Physical 
  Laboratory. 
  

  

  Aet. 
  XX 
  YI. 
  — 
  Epochs 
  and 
  Stages 
  of 
  the 
  Glacial 
  Period 
  / 
  by 
  

   Warren 
  Upham. 
  

  

  Renewed 
  studies 
  of 
  the 
  origin 
  and 
  order 
  in 
  age 
  of 
  our 
  

   Minnesota 
  drift 
  deposits 
  have 
  led 
  me 
  to 
  the 
  results 
  presented 
  in 
  

   the 
  following 
  table, 
  which 
  I 
  think 
  will 
  contribute 
  toward 
  a 
  

   reconciliation 
  and 
  harmony 
  of 
  the 
  lately 
  opposing 
  doctrines 
  

   (1) 
  of 
  unity 
  and 
  (2) 
  of 
  duality 
  or 
  greater 
  complexity 
  of 
  the 
  Ice 
  

   age. 
  Unity 
  or 
  continuity 
  of 
  our 
  Pleistocene 
  glaciation, 
  with 
  

   fluctuations 
  of 
  the 
  ice 
  margin, 
  much 
  greater 
  in 
  the 
  interior 
  of 
  

   the 
  continent 
  than 
  eastward, 
  appears 
  to 
  me 
  the 
  most 
  acceptable 
  

   view 
  and 
  statement, 
  when 
  the 
  whole 
  period 
  and 
  the 
  whole 
  

   drift-bearing 
  area 
  are 
  considered. 
  The 
  evidences 
  of 
  a 
  recession 
  

   of 
  the 
  ice- 
  sheet 
  in 
  Minnesota 
  about 
  two 
  hundred 
  miles 
  backward 
  

   from 
  the 
  nearest 
  portions 
  of 
  its 
  former 
  boundary, 
  followed 
  by 
  

   growth 
  again 
  nearly 
  to 
  its 
  previous 
  limits, 
  are 
  to 
  be 
  found 
  in 
  

   The 
  Geology 
  of 
  Minnesota, 
  final 
  report, 
  volumes 
  I 
  (1884) 
  and 
  

   II 
  (1888), 
  by 
  index 
  references 
  for 
  " 
  Interglacial 
  formations, 
  

   drainage 
  and 
  water-courses," 
  etc. 
  

  

  The 
  two 
  stages 
  of 
  growth 
  of 
  the 
  ice-sheet 
  may 
  have 
  been 
  

   due, 
  aside 
  from 
  their 
  principal 
  dependence 
  on 
  the 
  high 
  eleva- 
  

   tion 
  of 
  the 
  land, 
  to 
  the 
  last 
  two 
  passages 
  in 
  the 
  precession 
  of 
  

   the 
  equinoxes, 
  with 
  accompanying 
  nutation, 
  bringing 
  the 
  win- 
  

   ters 
  of 
  the 
  northern 
  hemisphere 
  in 
  aphelion 
  about 
  30,000 
  years 
  

   ago 
  and 
  again 
  about 
  10,000 
  years 
  ago. 
  The 
  intermediate 
  time 
  

   of 
  the 
  earth's 
  northern 
  winters 
  in 
  perihelion 
  would 
  be 
  the 
  stage 
  

   of 
  great 
  retreat 
  of 
  the 
  ice 
  margin 
  in 
  the 
  upper 
  Mississippi 
  

   region 
  ; 
  but 
  eastward, 
  from 
  Ohio 
  to 
  the 
  Atlantic 
  coast, 
  there 
  

   appears 
  to 
  have 
  been 
  little 
  glacial 
  oscillation.* 
  This 
  explana- 
  

   tion 
  accords 
  with 
  Prof. 
  E". 
  H. 
  Winchell's 
  computations 
  from 
  

   the 
  rate 
  of 
  recession 
  of 
  the 
  falls 
  of 
  v 
  St. 
  Anthony 
  for 
  the 
  Post- 
  

   glacial 
  or 
  Pecent 
  period, 
  f 
  and 
  with 
  his 
  estimate 
  of 
  the 
  duration 
  

  

  * 
  J. 
  D. 
  Dana, 
  this 
  Journal, 
  IN, 
  vol. 
  xlvi, 
  pp. 
  827-330, 
  Nov., 
  1893. 
  

  

  f 
  Geol. 
  and 
  Xat. 
  Hist. 
  Survey 
  of 
  Minnesota, 
  Fifth 
  Ann. 
  Rep. 
  for 
  1876, 
  pp. 
  175- 
  

   189; 
  Final 
  Report, 
  vol. 
  ii. 
  1888, 
  pp. 
  313-341, 
  with 
  fifteen 
  plates 
  (views 
  showing 
  

   recent 
  changes 
  of 
  the 
  falls 
  of 
  St. 
  Anthony, 
  and 
  maps). 
  Quart. 
  Jour. 
  Geol. 
  Soc, 
  

   London, 
  vol. 
  xxxiv, 
  1878, 
  pp. 
  880-901. 
  

  

  