﻿488 
  T. 
  G. 
  B0NKEY 
  OE 
  THE 
  GLACTER-EKOSION 
  

  

  elusion 
  that 
  they 
  are 
  older 
  than, 
  and 
  have 
  not 
  been 
  much 
  affected 
  

   by, 
  the 
  vast 
  ice-stream 
  which 
  has 
  passed 
  above 
  them. 
  

  

  The 
  portions 
  of 
  the 
  Val 
  de 
  Lys 
  and 
  Val 
  d'Ayas 
  which 
  I 
  examined 
  

   did 
  not 
  offer 
  any 
  fresh 
  evidence 
  of 
  importance, 
  except 
  that 
  the 
  

   singularly 
  steep 
  head 
  of 
  the 
  latter 
  is 
  very 
  difficult 
  to 
  explain 
  on 
  

   any 
  theory 
  of 
  glacier-erosion. 
  

  

  Assuming, 
  then, 
  that 
  glaciers 
  can 
  excavate 
  such 
  rock-basins 
  as 
  

   that 
  of 
  Orta 
  and 
  of 
  Como, 
  which 
  is 
  1900 
  feet 
  deep, 
  how 
  are 
  we 
  to 
  

   explain 
  their 
  impotent 
  action 
  on 
  the 
  felsite 
  rocks 
  that 
  bar 
  the 
  one, 
  

   and 
  on 
  the 
  conglomerate 
  and 
  Molasse 
  that 
  bar 
  the 
  other 
  ? 
  how 
  to 
  

   explain 
  the 
  absence 
  of 
  all 
  approach 
  to 
  lake-basins 
  in 
  those 
  other 
  

   valleys 
  through 
  which 
  the 
  ice-streams 
  must 
  have 
  descended 
  from 
  a 
  

   yet 
  longer 
  period, 
  the 
  absence 
  of 
  any 
  but 
  comparatively 
  superficial 
  

   effects 
  over 
  all 
  the 
  rest 
  of 
  the 
  district 
  ? 
  No 
  alteration 
  in 
  the 
  mineral 
  

   character 
  of 
  the 
  strata 
  can 
  account 
  for 
  it. 
  The 
  Lake 
  of 
  Como 
  begins 
  

   in 
  crystalline 
  schists, 
  and 
  passes 
  without 
  much 
  marked 
  change 
  in 
  

   width 
  and 
  depth 
  through 
  various 
  sedimentary 
  rocks. 
  The 
  Lake 
  of 
  

   Orta 
  also 
  appears 
  to 
  lie 
  in 
  rocks 
  no 
  softer 
  than 
  those 
  of 
  Val 
  Strona 
  

   and 
  Yal 
  Sesia. 
  Did 
  time 
  allow, 
  I 
  might 
  appeal 
  to 
  other 
  evidence 
  on 
  

   the 
  Lago 
  Maggiore 
  and 
  the 
  Lago 
  di 
  Lugano, 
  the 
  form 
  of 
  which 
  appears 
  

   to 
  me 
  wholly 
  inexplicable 
  on 
  any 
  theory 
  of 
  glacier 
  erosion 
  ; 
  but 
  I 
  

   think 
  I 
  have 
  done 
  enough 
  to 
  show 
  that, 
  even 
  if 
  this 
  be 
  retained 
  as 
  the 
  

   explanation 
  of 
  some 
  of 
  the 
  upland 
  tarns 
  in 
  the 
  Alps, 
  it 
  is 
  wholly 
  

   inadequate 
  to 
  explain 
  the 
  great 
  basins 
  of 
  the 
  Italian 
  lakes. 
  These 
  

   and 
  their 
  valleys 
  appear 
  to 
  be 
  much 
  older 
  than 
  the 
  ice-age, 
  and 
  to 
  

   have 
  been 
  but 
  little 
  modified 
  during 
  the 
  period 
  of 
  maximum 
  exten- 
  

   sion 
  of 
  the 
  glaciers. 
  

  

  The 
  theory 
  which 
  I 
  propounded 
  on 
  a 
  former 
  occasion*, 
  of 
  irregular 
  

   movements 
  of 
  upheaval 
  and 
  subsidence 
  along 
  lines 
  athwart 
  the 
  valleys, 
  

   can 
  be 
  applied 
  to 
  these 
  lakes. 
  There 
  is 
  nothing 
  in 
  the 
  stratigraphy 
  

   of 
  the 
  district 
  opposed 
  to 
  it 
  ; 
  so 
  far 
  as 
  the 
  evidence 
  goes, 
  it 
  is 
  rather 
  

   favourable 
  to 
  it. 
  The 
  anomalous 
  form 
  of 
  the 
  Lake 
  of 
  Lugano, 
  the 
  

   position 
  of 
  the 
  deepest 
  parts 
  of 
  the 
  Lake 
  of 
  Como 
  f 
  , 
  the 
  variation 
  in 
  

   the 
  level 
  of 
  its 
  floor, 
  all 
  agree 
  very 
  well 
  with 
  such 
  a 
  theory 
  ; 
  as 
  does 
  

   the 
  slight 
  slope 
  that 
  we 
  must 
  assign 
  to 
  the 
  preglacial 
  valley, 
  compared 
  

   with 
  the 
  fall 
  of 
  the 
  other 
  neighbouring 
  valleys. 
  The 
  extraordinary 
  

   extension 
  of 
  the 
  old 
  Como 
  lake 
  up 
  the 
  Yal 
  Telline 
  also 
  agrees 
  very 
  well 
  

   with 
  this, 
  and 
  induces 
  us 
  to 
  suppose 
  that 
  the 
  existence 
  of 
  these 
  lakes 
  

   is 
  due 
  to 
  the 
  relative 
  subsidence 
  of 
  a 
  district 
  included 
  roughly 
  between 
  

   one 
  line 
  drawn 
  through 
  Chiavenna 
  from 
  east 
  to 
  west, 
  and 
  another 
  

   through 
  Lecco, 
  Como, 
  and 
  Arona. 
  This 
  subsidence 
  became 
  less 
  going 
  

   westward 
  ; 
  but 
  it 
  may 
  have 
  affected 
  an 
  area 
  some 
  distance 
  further 
  to 
  

   the 
  east. 
  

  

  Discussion. 
  

  

  Mr. 
  Dkew 
  differed 
  from 
  the 
  author, 
  and 
  thought 
  that 
  glaciers 
  

   coming 
  from 
  different 
  directions 
  might 
  have 
  produced 
  single 
  results 
  

   at 
  certain 
  points. 
  

  

  * 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xxix. 
  p. 
  382. 
  

  

  t 
  See 
  an 
  important 
  paper 
  by 
  Mr. 
  Ball, 
  Geol. 
  Mag. 
  vol. 
  viii. 
  p. 
  359. 
  

  

  