﻿27S 
  

  

  J". 
  S. 
  GARDNER 
  ON 
  TflE 
  GEOLOGY 
  OF 
  MADEIRA. 
  

  

  the'Curral, 
  and 
  continues 
  thence 
  as 
  a 
  narrow 
  gorge 
  to 
  the 
  sea. 
  It 
  

   is 
  remarkable 
  that 
  this 
  gigantic 
  depression 
  occupies 
  an 
  almost 
  iden- 
  

   tical 
  relative 
  position 
  in 
  Madeira 
  that 
  the 
  Peak 
  occupies 
  in 
  Tenerifte, 
  

   which 
  would, 
  moreover, 
  if 
  deprived 
  of 
  its 
  central 
  cone, 
  strougly 
  

   resemble 
  it 
  in 
  size 
  and 
  in 
  configuration. 
  

  

  Sir 
  Charles 
  Lyell, 
  however, 
  in 
  the 
  ' 
  Elements 
  of 
  Geology/ 
  18<J5, 
  

   expressed 
  the 
  opinion 
  that 
  the 
  Curral 
  was 
  not 
  an 
  ancient 
  crater, 
  but 
  

   a 
  valley 
  of 
  erosion 
  like 
  the 
  Little 
  Curral 
  or 
  the 
  neighbouring 
  Serra 
  

   d'Agua 
  ; 
  but 
  as 
  this 
  view 
  seems 
  to 
  have 
  been 
  omitted 
  in 
  later 
  editions, 
  

   it 
  is 
  possible 
  that 
  Sir 
  Charles 
  himself 
  saw 
  reason 
  to 
  change 
  it. 
  There 
  

   can 
  be 
  no 
  doubt, 
  in 
  fact, 
  that 
  it 
  is 
  actually 
  a 
  magnificent 
  example 
  of 
  

   what 
  Scrope 
  has 
  described 
  as 
  the 
  basal 
  wreck 
  of 
  a 
  volcanic 
  mountain 
  

   blown 
  into 
  the 
  air 
  by 
  some 
  paroxysm 
  of 
  peculiar 
  violence 
  and 
  

   persistence. 
  Its 
  almost 
  perpendicular 
  walls 
  reveal 
  sections 
  of 
  at 
  

   least 
  3500 
  feet 
  of 
  stratified 
  basalts 
  and 
  tuffs, 
  through 
  which 
  enormous 
  

   vertical 
  trachytic 
  dykes 
  radiate 
  as 
  if 
  from 
  a 
  common 
  centre. 
  The 
  

   edge 
  of 
  the 
  former 
  crater, 
  though 
  fretted 
  into 
  mountain-peaks, 
  

   preserves 
  in 
  many 
  places 
  the 
  beds 
  of 
  scoriae 
  of 
  the 
  ancient 
  cone, 
  

   remains 
  of 
  which 
  arc 
  seen 
  sloping 
  outwardly 
  on 
  all 
  sides, 
  at 
  the 
  usual 
  

   augles 
  of 
  rest 
  assumed 
  by 
  volcanic 
  ash. 
  The 
  radiating 
  dykes 
  have 
  

   been 
  weathered 
  on 
  the 
  eastern 
  side 
  into 
  the 
  inaccessible 
  and 
  fantastic 
  

   summits 
  of 
  the 
  Torres, 
  while 
  on 
  the 
  north 
  and 
  west 
  are 
  the 
  more 
  

   perfect 
  and 
  imposing 
  masses 
  of 
  lvuivo 
  and 
  Canario. 
  There 
  is 
  no 
  

   other 
  trace 
  of 
  any 
  great 
  crater 
  throughout 
  the 
  island 
  : 
  and 
  it 
  seems 
  

   perfectly 
  obvious 
  that 
  during 
  the 
  trachytic 
  period, 
  at 
  least, 
  this 
  was 
  

   the 
  main 
  vent 
  whence 
  the 
  lavas 
  and 
  scoriae 
  were 
  poured 
  out. 
  The 
  

   two 
  craters 
  mentioned 
  by 
  Lyell, 
  one 
  atS. 
  Antonio 
  da 
  Serra, 
  towards 
  

   the 
  east 
  end 
  of 
  the 
  island, 
  and 
  the 
  other 
  to 
  the 
  west, 
  on 
  the 
  edge 
  of 
  

   an 
  elevated 
  plateau, 
  the 
  Paiil 
  da 
  Serra, 
  are 
  quite 
  insignificant, 
  and 
  

   were 
  probably 
  only 
  formed 
  when 
  volcanic 
  activity 
  was 
  declining. 
  

  

  The 
  Fosstllfcrous 
  Limestone 
  of 
  Madeira. 
  

  

  It 
  appears 
  extremely 
  probable 
  that 
  the 
  limestone 
  deposit 
  to 
  which 
  

   I 
  have 
  already 
  referred 
  is 
  the 
  only 
  one 
  now 
  exposed 
  in 
  the 
  main 
  

   island 
  of 
  Madeira 
  ; 
  for 
  limestone 
  is 
  in 
  request 
  in 
  building 
  the 
  great 
  

   irrigation 
  works 
  so 
  long 
  in 
  progress, 
  yet 
  it 
  is 
  now 
  wholly 
  imported 
  

   from 
  Porto 
  Santo, 
  where 
  it 
  is 
  the 
  fundamental 
  formation 
  of 
  one 
  of 
  the 
  

   smaller 
  islets 
  of 
  the 
  Porto 
  Santo 
  group. 
  This 
  deposit 
  is 
  situated 
  on 
  

   the 
  left 
  hand 
  in 
  ascending 
  the 
  gorge 
  of 
  Sao 
  Yicenti, 
  in 
  a 
  ravine 
  known 
  

   as 
  the 
  Achada 
  do 
  Eurtado, 
  13r>0 
  feet 
  above 
  the 
  sea, 
  and 
  consists 
  of 
  

   a 
  tufaceous 
  conglomerate 
  with 
  coralline 
  limestone 
  and 
  other 
  remains 
  

   of 
  a 
  marine 
  fauna. 
  The 
  upper 
  part 
  is 
  crystalline 
  and 
  compact, 
  with 
  

   indistinct 
  traces 
  of 
  Mollusca, 
  though 
  Pryozoa 
  are 
  sometimes 
  

   beautifully 
  weathered 
  out, 
  especially 
  towards 
  the 
  top. 
  Lower 
  down 
  

   it 
  is 
  less 
  compact 
  and 
  contains 
  shells, 
  corals, 
  and 
  a 
  large 
  Clypeaster. 
  

   Some 
  80 
  feet 
  lower 
  clown 
  is 
  a 
  kiln, 
  and 
  25 
  feet 
  below 
  this 
  again 
  is 
  

   a 
  breccia 
  of 
  rounded 
  boulders 
  and 
  fragments 
  containing 
  bivalves 
  and 
  

   spines 
  of 
  Echinodermata. 
  The 
  limestone 
  is 
  only 
  partially 
  exposed 
  on 
  

  

  