744 
SIE  CHAELES  LTELL  ON  THE  STEHCTTEE  OE  LAVAS 
c,  fig.  16,  and  those  immediately  below  in  the  same  cliflF  exposed  in  Cmigghiuni  and 
other  rocky  promontories,  could  not  have  retained  the  undisturbed  and  nearly  hori- 
zontal position  which  they  now  enjoy.] 
Comparison  of  the  twofold  axis  of  Etna  with  that  of  Madeira. 
The  analogous  structure  of  the  island  of  Madeira,  where  there  is  also  a twofold  axis 
of  eruption,  encourages  me  to  believe  that  the  mode  of  origin  above  ascribed  to  Moimt 
Etna  is  correct.  My  fellow-traveller,  Mr.  Haktung,  and  I found  in  1853-54  that  the 
lavas  have  flowed  in  Madeira  chiefly  from  one  main  axis  or  chain  of  volcanic  vents,  thirty 
miles  in  length,  and  have  overwhelmed  the  materials  eructed  from  a shorter  chain  which 
runs  parallel  to  the  other.  In  the  case  of  the  Sicilian  Mountain,  we  have  a larger  cone 
overwhelming  and  burying  a smaller  one ; whereas  in  the  case  of  the  Atlantic  island, 
we  have  a linear  series  of  volcanic  cones,  rising  at  some  points  to  a height  of  6000  feet, 
whence  lava  and  scoriae  have  been  poured,  which  have  not  only  filled  up  the  intercolline 
space,  but  have  so  overtopped  the  secondary  chain,  as  to  bury  it  under  a stratified  mass 
2000  feet  in  thickness.  In  both  cases  there  is  a difference  in  the  mineral  composition 
of  the  older  and  newer  lavas,  although  the  order  of  their  relative  age  is  reversed,  the 
trachytic  lavas  in  Madeira  being  the  more  modern  of  the  two.  In  both  volcanic 
districts  there  is  a deep  crateriform  valley,  which  to  a certain  extent  cuts  through  the 
products  of  the  two  axes,  for  such  is  the  celebrated  Curral  in  Madeira,  4000  feet  deep 
towards  its  upper  extremity  and  3000  feet  deep,  where  it  intersects  the  secondaiy  chain 
at  a distance  of  two  miles  from  the  main  axis.  Nor  does  the  analogy  end  here ; for  in 
the  same  manner  as  we  see  on  the  west  side  of  Mount  Etna  towards  Bronte  (see  fig.  15, 
p.  740),  where  no  lateral  cone  like  that  of  Trifoglietto  interfered  with  the  regular 
growth  of  the  volcano,  that  the  lavas  slope  steeply  from  the  elevated  platform  down  to 
the  base,  so  in  Madeira,  on  the  north  side  of  the  main  axis,  the  lavas  have  a uniform 
steep  inclination  to  the  sea,  there  being  on  that  side,  as  the  numerous  raAnes  show  us, 
no  lateral  buried  chain  which  could  present  any  obstruction, — no  intercolline  space  like 
that  on  the  southern  slope  to  cause  the  lavas  to  become  horizontal*. 
It  follows  as  a corollary  from  the  views  above  set  forth,  that  in  the  cone-making  pro- 
cess the  force  of  upheaval  from  below,  if  it  has  acted  at  all,  has  only  exerted  a subordi- 
nate and  possibly  a very  local  influence,  and  the  admission  of  a double  axis  for  Etna 
draws  with  it  the  abandonment  of  the  “elevation-crater”  hypothesis;  for  however  con- 
ceivable it  may  be  that  one  cone  of  eruption  should  envelope  and  biu-y  an  adjoining 
cone  of  eruption,  it  is  ob\iously  impossible  that  one  cone  of  upheaval  should  mantle 
round,  overwhelm  and  bury  another  cone  of  upheaval. 
* See  section  of  Madeira,  ‘ Manual  of  Geology,’  chap.  xxix.  fig.  653,  p.  517,  5th  edit.,  -where  A is  the 
central  axis,  s E the  intercoUine  space  filled  up,  and  cf  the  secondary  chain  buried  under  2000  feet  of  lava 
and  scoriae. 
