﻿J. 
  S. 
  GARDNER 
  ON 
  THE 
  GEOLOGY 
  OF 
  MADEIRA. 
  277 
  

  

  28. 
  The 
  Geology 
  of 
  Madeira. 
  By 
  J. 
  Starkie 
  Gardner, 
  Esq., 
  F.G.S. 
  

   (Read 
  March 
  8, 
  1882.) 
  

  

  The 
  geology 
  of 
  Madeira 
  has 
  already 
  formed 
  the 
  subject 
  of 
  three 
  

   communications 
  to 
  this 
  Society, 
  and 
  the 
  present 
  notes 
  are 
  therefore 
  

   brief. 
  

  

  The 
  island, 
  it 
  is 
  well 
  known, 
  is 
  almost 
  wholly 
  composed 
  of 
  basaltic 
  

   rocks, 
  which 
  rise 
  more 
  or 
  less 
  precipitously 
  from 
  the 
  sea 
  and 
  culmi- 
  

   nate 
  in 
  a 
  central 
  ridge 
  of 
  some 
  5000 
  or 
  6000 
  feet 
  in 
  height. 
  

   Including 
  the 
  Desertas, 
  which 
  are 
  separated 
  by 
  a 
  narrow 
  channel, 
  

   only 
  400 
  feet 
  in 
  depth, 
  its 
  extreme 
  length 
  may 
  be 
  roughly 
  stated 
  at 
  

   50 
  miles, 
  and 
  its 
  greatest 
  breadth 
  at 
  17 
  miles 
  ; 
  yet 
  so 
  broken 
  is 
  the 
  

   surface 
  that 
  some 
  250 
  miles 
  must 
  be 
  traversed 
  in 
  order 
  to 
  become 
  well 
  

   acquainted 
  with 
  its 
  varying 
  scenery. 
  The 
  oldest 
  of 
  the 
  basalts 
  are 
  

   considered 
  to 
  have 
  been 
  of 
  submarine 
  formation, 
  and 
  the 
  only 
  palseon- 
  

   tological 
  evidence 
  as 
  to 
  their 
  age 
  consists 
  of 
  a 
  small 
  patch 
  of 
  coralline 
  

   limestone 
  included 
  towards 
  the 
  north-west 
  of 
  the 
  isle, 
  and 
  referred 
  to 
  

   the 
  Upper 
  Miocene 
  ; 
  while 
  a 
  hitherto 
  undescribed 
  bed, 
  with 
  fossil 
  

   plants, 
  shows 
  that 
  volcanic 
  activity 
  was 
  continued 
  until 
  the 
  most 
  

   recent 
  period. 
  There 
  are, 
  however, 
  no 
  traces 
  of 
  either 
  hot 
  springs, 
  

   fumaroles, 
  or 
  sulphur 
  deposits, 
  though 
  some 
  of 
  the 
  lava-streams 
  appear 
  

   as 
  unweathered 
  as 
  when 
  newly 
  formed 
  ; 
  and 
  immediately 
  west 
  of 
  

   Funchal 
  there 
  are 
  parasitic 
  cones 
  so 
  little 
  altered 
  that 
  it 
  seems 
  difficult 
  

   to 
  imagine 
  them 
  completely 
  extinct. 
  Madeira, 
  since 
  its 
  discover}', 
  

   has 
  been 
  profoundly 
  quiescent 
  ; 
  if 
  any 
  movement 
  takes 
  place, 
  it 
  is 
  

   one 
  of 
  slow 
  elevation. 
  

  

  The 
  chief 
  feature 
  about 
  the 
  basalt 
  is 
  the 
  enormous 
  number 
  of 
  

   dykes 
  which 
  riddle 
  them 
  in 
  every 
  direction 
  ; 
  and 
  in 
  this 
  respect 
  they 
  

   are 
  in 
  striking 
  contrast 
  to 
  the 
  unbroken 
  sheets 
  which 
  characterize 
  

   Iceland 
  and 
  the 
  Faroes. 
  Every 
  cliff 
  and 
  precipice 
  shows 
  sheet 
  after 
  

   sheet 
  of 
  lava, 
  not 
  continuous 
  or 
  parallel 
  for 
  long 
  distances, 
  but 
  

   variable 
  in 
  thickness, 
  interstratified 
  with 
  tuffs, 
  scoria?, 
  and 
  bright 
  

   brick-coloured 
  earths, 
  and 
  cut 
  through 
  in 
  every 
  direction 
  by 
  innu- 
  

   merable 
  dykes. 
  These 
  dykes, 
  weathered 
  into 
  mountain-tops 
  of 
  the 
  

   most 
  fantastic 
  forms, 
  give 
  a 
  grandeur 
  and 
  ruggedness 
  to 
  the 
  scenery 
  

   far 
  beyond 
  any 
  thing 
  I 
  have 
  met 
  with 
  in 
  other 
  basaltic 
  districts, 
  while 
  

   the 
  thorough 
  immunity 
  from 
  glacial 
  action 
  has 
  left 
  the 
  sides 
  of 
  the 
  

   gorges 
  and 
  ravines 
  in 
  almost 
  vertical 
  precipices. 
  

  

  Occupying 
  a 
  nearly 
  central 
  position 
  in 
  the 
  island 
  is 
  the 
  Curral, 
  a 
  

   deep 
  horseshoe-shaped 
  valley, 
  more 
  than 
  four 
  miles 
  in 
  diameter 
  and 
  

   with 
  its 
  bed 
  raised 
  2500 
  feet 
  above 
  the 
  sea. 
  It 
  is 
  almost 
  encircled 
  

   by 
  precipitous 
  walls, 
  which 
  tower 
  3000 
  feet 
  above 
  it, 
  and 
  embrace 
  the 
  

   loftiest 
  peaks 
  (Pico 
  Euivo 
  6050 
  feet, 
  Sidrao 
  5500, 
  the 
  Torres 
  5980, 
  

   &c), 
  from 
  which 
  radiate 
  the 
  principal 
  mountain- 
  chains 
  in 
  the 
  

   island. 
  The 
  walls 
  are 
  breached 
  to 
  the 
  south 
  and 
  form 
  the 
  outlet 
  of 
  

   a 
  torrent, 
  the 
  Ribeira 
  dos 
  Soccorridos. 
  This 
  valley, 
  at 
  first 
  narrow, 
  

   widens 
  into 
  an 
  ellipse, 
  similar 
  to, 
  but 
  in 
  every 
  respect 
  less 
  grand 
  than, 
  

  

  Q.J. 
  G. 
  IS. 
  No. 
  151. 
  v 
  

  

  