706 
SIE  CHAELES  LYELL  OX  THE  STEUCTTEE  OE  LAVAS 
“ Eecherches  sur  le  Mont  Etna,”  says  (in  the  same  volume,  p.  184)  that  the  solid  beds 
of  ancient  lava  in  the  Val  del  Bove  (which  dip  often  at  28°  and  upwards)  resemble  those 
portions  only  of  a modem  current  which  have  flowed  over  ground  almost  flat,  or  not 
sloping  at  an  angle  of  more  than  3°.  And  again,  speaking  of  the  Vesuvian  lavas  seen 
in  the  Fosso  Grande,  he  observes,  “ they  attain  a thickness  of  4 or  5 metres  when  hori- 
zontal, but  are  thin  on  slopes  of  5°  or  6°”  (ib.  p.  169). 
From  these  premises  it  has  been  logically  inferred  that  almost  all  volcanic  mountains, 
whether  active  or  extinct,  have  acquired  their  present  conical  form,  not  by  eraption,  or  the 
reiterated  outpouring  of  lava  and  outthrow  of  ashes  from  one  or  more  central  craters, 
but  by  upheaval,  or  the  upward  and  outward  pressure  of  gases  and  fluid  matter  rising 
from  below  and  disturbing  the  position  of  lavas  and  tuffs  preriously  horizontal.  For 
wherever  we  obtain  access  to  sections  displaying  the  internal  structure  of  large  volcanic 
cones,  we  find  a series  of  stony  layers  several  feet  or  yards  in  thickness,  inclined  at 
angles  of  from  10°  to  30°,  and  sometimes,  though  rarely,  more  than  35°;  and  if  it  be 
true  that  the  law  above  laid  down  does  really  govern  the  congelation  of  melted  matter, 
the  whole  excess  of  dip  above  2°,  or  according  to  some  authorities  above  5°  or  6°,  must 
have  been  due  to  mechanical  force,  or  to  some  cause  capable  of  altermg  the  original 
position  of  the  beds.  Accordingly  the  following  hypothesis  has  been  suggested : — A dense 
series  of  horizontal  or  nearly  horizontal  sheets  of  lava  and  scorise  having  been  deposited, 
a force  operating  from  below  and  exerting  a pressure  both  upwards  and  outwards  from 
a central  axis  towards  all  points  of  the  compass,  has  uplifted  suddenly  the  whole  strati- 
fied mass  and  made  it  assume  a conical  form,  giring  rise  at  the  same  time,  in  many  cases, 
to  a wide  and  deep  circular  opening  at  the  top  of  the  cone,  an  opening  called  by  the 
advocates  of  this  hypothesis  a “ Crater  of  Elevation.” 
When  1 first  visited  Vesuvius  and  Etna  in  the  autumn  of  1828,  I was  so  struck  with 
the  analogy  of  the  ancient  and  modern  portions  of  those  volcanos,  that  it  appeared  to 
me  in  the  highest  degree  unphilosophical  to  imagine  that  they  could  owe  their  form  to 
operations  differing  in  kind  or  degree  from  those  witnessed  during  ordinary  eruptions. 
Seeing  that  the  injection  of  lava  into  fissures,  and  the  consequent  formation  of  dikes, 
had  occurred  in  the  most  modern  part  of  the  cone  of  Vesuvius,  I included  dikes  among 
the  products  of  ordinary  eruptions,  and  inferred  that  an  adequate  lapse  of  years  alone 
was  requhed  to  reproduce  gradually  such  volcanic  mountains  as  had  been  formed  in  the 
course  of  previous  ages.  My  grounds  for  adopting  such  an  opinion  were  stated  fully  in 
the  first  volume  of  my  ‘Principles  of  Geology,’  published  in  1830  (see  pp.  345,  394. 
chaps.  20  and  22),  in  which,  as  well  as  in  the  third  volume  published  in  1833  (p.  84),  1 
objected  to  the  theory  of  Elevation- craters,  first  propounded  by  the  late  Baron  Leopold 
VON  Bucii. 
Mr.  ScEOPE,  in  his  ‘Considerations  on  Volcanos’  (1825,  p.  156,  4),  had  previously 
advocated  the  same  views,  and  had  dwelt  emphatically  on  the  increase  both  in  solidity 
and  bulk  imparted  to  volcanic  cones,  by  the  injection  of  lava  into  fissures  in  the  heart 
of  the  mountain.  He  had  moreover  admitted,  in  a paper  read  to  the  Geological  Society 
