﻿THEORY 
  OF 
  LAKE-BASINS. 
  485 
  

  

  elude 
  that 
  the 
  principal 
  features 
  of 
  the 
  scene 
  are 
  preglacial, 
  and 
  

   that 
  the 
  glacier 
  has 
  only 
  slightly 
  modified 
  them. 
  

  

  Just 
  below 
  Chiavenna 
  the 
  true 
  basin 
  of 
  the 
  Lake 
  of 
  Como 
  begins, 
  

   at 
  the 
  union 
  of 
  the 
  Mera 
  and 
  the 
  Lira 
  (which 
  latter 
  is 
  orographic- 
  

   ally 
  the 
  principal 
  valley) 
  ; 
  for 
  a 
  broad 
  level 
  strath 
  extends 
  all 
  the 
  

   way 
  down 
  to 
  the 
  waters 
  of 
  the 
  Lago 
  de 
  Mezzola, 
  and 
  Chiavenna 
  

   itself 
  is 
  less 
  than 
  350 
  feet 
  above 
  the 
  level 
  of 
  the 
  lake. 
  Ice-worn 
  

   rocks 
  may 
  be 
  traced 
  on 
  the 
  mountain- 
  sides 
  from 
  at 
  least 
  2000' 
  above, 
  

   down 
  to 
  the 
  very 
  edge 
  of 
  the 
  meadows. 
  

  

  Is 
  it 
  not 
  then 
  rather 
  perplexing 
  that 
  these 
  glaciers 
  of 
  the 
  Lira 
  and 
  

   the 
  Mera, 
  which 
  singly 
  have 
  only 
  slightly 
  modified 
  the 
  valleys 
  in 
  

   which 
  they 
  flowed, 
  should, 
  on 
  their 
  union, 
  have 
  been 
  suddenly 
  gifted 
  

   with 
  such 
  great 
  excavating 
  power 
  as 
  to 
  scoop 
  out 
  from 
  hard 
  schist 
  

   (micaceous 
  and 
  hornblendic) 
  and 
  granite 
  the 
  open 
  valley 
  above 
  the 
  

   Lago 
  di 
  Como 
  ? 
  And 
  if 
  they 
  have 
  done 
  this, 
  how 
  are 
  we 
  to 
  explain 
  

   the 
  comparatively 
  slight 
  effect 
  produced 
  by 
  the 
  junction 
  of 
  the 
  broad 
  

   Val 
  Telline, 
  which 
  must 
  have 
  brought 
  down 
  an 
  immense 
  affluent 
  of 
  

   ice. 
  About 
  two 
  fifths 
  of 
  the 
  present 
  Lake 
  of 
  Como 
  lies 
  in 
  mica-schists, 
  

   the 
  remainder 
  (including 
  its 
  two 
  arms) 
  in 
  sedimentary 
  rocks, 
  fol- 
  

   lowing 
  one 
  another 
  in 
  ascending 
  order, 
  and 
  (according 
  to 
  Yon 
  Hauer) 
  

   members 
  of 
  the 
  Silurian 
  ('?), 
  Trias, 
  and 
  Rhaetic 
  formations. 
  The 
  

   strata, 
  as 
  a 
  rule, 
  dip 
  towards 
  the 
  S., 
  often 
  at 
  a 
  high 
  angle 
  ; 
  but 
  there 
  

   are 
  one 
  or 
  two 
  rolls, 
  as 
  near 
  Balbaniello 
  and 
  the 
  Val 
  Intelvi, 
  till 
  

   at 
  last 
  (in 
  the 
  Como 
  arm) 
  they 
  roll 
  gradually 
  up 
  and 
  down, 
  and 
  

   finally 
  dip 
  sharply 
  away 
  from 
  the 
  fault 
  which 
  brings 
  the 
  Nagelflue 
  

   conglomerates 
  against 
  them. 
  

  

  This 
  arm 
  of 
  the 
  Como 
  lake 
  has 
  always 
  seemed 
  to 
  me 
  difficult 
  to 
  

   explain 
  on 
  any 
  theory 
  of 
  glacier 
  erosion 
  ; 
  the 
  low 
  promontory 
  of 
  

   Bellagio, 
  unless 
  the 
  valley 
  were 
  previously 
  well 
  defined, 
  seems 
  inade- 
  

   quate 
  to 
  split 
  a 
  glacier 
  into 
  two 
  such 
  divergent 
  arms 
  ; 
  and 
  its 
  form 
  

   does 
  not 
  suggest 
  that 
  it 
  has 
  been 
  exposed 
  to 
  any 
  such 
  tremendous 
  

   pressure 
  as 
  it 
  would 
  have 
  undergone 
  if 
  this 
  had 
  been 
  the 
  case. 
  This 
  

   Como 
  arm, 
  it 
  is 
  well 
  known, 
  has 
  no 
  outlet, 
  and 
  is 
  chiefly 
  cut 
  off 
  

   from 
  the 
  plain 
  of 
  Lombardy 
  by 
  a 
  low 
  range 
  of 
  hills 
  composed 
  to 
  a 
  

   large 
  extent 
  of 
  Nagelflue, 
  the 
  pebbles 
  being 
  mostly 
  metamorphic 
  

   rock, 
  quartzites, 
  schists, 
  and 
  the 
  porphyritic 
  gneissic 
  and 
  granitoid 
  

   rocks 
  common 
  in 
  the 
  Yal 
  Bregaglia, 
  generally 
  well 
  rolled, 
  not 
  

   usually 
  exceeding 
  about 
  a 
  foot 
  in 
  diameter. 
  They 
  lie 
  in 
  a 
  grey 
  

   sandy 
  matrix 
  like 
  Molasse, 
  with 
  sandy 
  bands. 
  The 
  occurrence 
  of 
  the 
  

   gneissic 
  and 
  granitoid 
  pebbles 
  is 
  important, 
  as 
  showing 
  that 
  the 
  valley 
  

   of 
  the 
  Lira 
  was 
  defined 
  at 
  a 
  very 
  early 
  period. 
  

  

  This 
  range 
  of 
  hills 
  is 
  very 
  peculiar 
  in 
  shape. 
  It 
  is 
  a 
  sharp 
  ridge 
  

   with 
  steep 
  slopes 
  on 
  both 
  sides, 
  its 
  tops 
  in 
  places 
  being 
  almost 
  an 
  

   arete. 
  The 
  old 
  glacier 
  has 
  indeed 
  passed 
  over 
  it 
  ; 
  for 
  on 
  my 
  ascent 
  

   to 
  the 
  tower 
  (Castello 
  Baradello) 
  which 
  is 
  perched 
  on 
  the 
  crest, 
  I 
  saw, 
  

   in 
  a 
  face 
  of 
  rock 
  near 
  S. 
  Carpofaro, 
  that 
  the 
  pebbles 
  had 
  been 
  planed 
  

   level 
  with 
  the 
  matrix, 
  and 
  a 
  few 
  of 
  them 
  yet 
  retained 
  strise. 
  More- 
  

   over 
  in 
  several 
  other 
  places 
  the 
  characteristic 
  contours 
  of 
  ice-worn 
  

   rocks 
  could 
  yet 
  be 
  recognized, 
  and 
  perched 
  blocks 
  could 
  be 
  seen 
  

   dotted 
  here 
  and 
  there 
  ; 
  indeed 
  a 
  large 
  one 
  (of 
  the 
  usual 
  granitoid 
  

  

  