WHICH  ha^t:  consolidated  on  steep  slopes. 
7G9 
by  the  sinking  of  the  ground,  and  again  deepened,  according  to  M.  E.  de  Beaumont,  by 
a subsidence  in  1832.  I also  saw  at  a still  higher  level,  near  the  Philosopher’s  Tower,  a 
fosse-like  depression,  already  half-tilled  up  by  recent  showers  of  ashes  which  is  known  to 
have  originated  during  the  same  eruption  of  1832.  The  great  rent  of  Mascalucia,  formed 
in  1381,  is  still  open,  a mile  in  length  and  from  20  to  30  feet  deep.  Another  fissure, 
6 feet  broad  and  of  unknown  depth,  was  formed  in  the  plain  of  San  Lio  in  1G69,  and  is 
said  to  have  been  12  miles  long,  reaching  to  near  the  summit  of  Etna.  Similar  open- 
ings on  the  steep  parts  of  a cone  might  easily  become  water-courses,  and  give  passage  to 
floods  during  the  winter’s  rain  and  the  melting  of  the  snow,  and  these  might  gradually 
deepen  and  uiden  such  fissures.  But  it  is  also  fair  to  speculate  on  another  very  powerful 
agency,  which  in  certain  phases  of  volcanic  activity,  as  Mr.  Sceope  has  frequently  sug- 
gested to  me,  would  scarcely  fail  to  come  into  play, — I mean  paroxysmal  explosions, 
like  that  of  Vesmius  in  the  year  79,  wholly  unaccompanied  by  any  outpouring  of  lava. 
There  were  repeated  eruptions  of  this  kind  in  the  history  of  Vesmius  for  centuries  after 
the  year  79;  and  if  a great  explosion  happened  to  be  lateral  instead  of  central,  or  on  the 
slope  instead  of  the  highest  point  of  a cone,  the  new  chasm  being  commanded  by  higher 
ground  or  by  the  region  of  snow,  floods  of  water  would  at  certain  seasons  sweep  down 
into  it,  and  might  increase  its  dimensions.  To  account  for  the  position  of  so  great  a 
ca’vity  on  one  side  only  of  a cone,  we  may,  in  the  case  of  Etna,  imagine  a connexion 
between  the  Yal  del  Bove  and  the  old  axis  of  Trifoglietto.  The  ancient  habitual  duct 
or  chimney,  situated  at  T in  the  wood-cut  map  (fig.  14,  p.  739),  may,  like  that  of  the 
ancient  YesuYus  after  being  plugged  up  for  ages,  have  again  given  passage  to  vast 
volumes  of  pent-up  gases  or  steam,  blowing  up  the  incumbent  lavas  of  Mongibello, 
which  had  filled  the  crater  and  overtopped  the  secondary  cone  (as  shown  in  fig.  15, 
p.  740).  Moreover,  the  accumulated  snow  and  ice,  and  consequently  the  action  of  run- 
ning water,  may  at  some  earlier  period  have  been  greater  in  the  higher  region,  when 
the  cone  of  Mongibello  was  larger  and  loftier,  before  its  truncation,  especially  if  the 
first  excavation  of  the  Yal  del  Bove  dates  as  far  back  as  the  close  of  the  glacial  period, 
or  when  the  Alpine  glaciers  reached  the  plains  of  the  Po ; for  at  that  time  the  climate 
of  a Sicilian  winter  could  hardly  fail  to  have  been  colder  than  now. 
Isolated  outliers  of  ancient  rock,  such  as  Einocchio  and  Musara,  are  striking  monu- 
ments of  waste,  helping  to  prove  the  former  continuity  of  the  northern  escarpment  of 
the  Yal  del  Bove  in  a southerly  direction ; but  unfortunately  for  the  geologist,  the  founda- 
tions of  these  outliers  are  so  much  concealed  by  recent  lava,  that  we  cannot  determine 
whether  the  removal  of  the  missing  rocks  was  brought  about  chiefiy  by  aqueous  agency, 
or  by  engulfment,  or  by  explosions,  or  by  a combination  of  two  or  more  of  these  causes. 
The  multitude  of  dikes  projecting  from  10  to  50  feet  above  the  general  level  of  the 
ground  in  every  part  of  the  escarpments,  shows  clearly  to  what  an  extent  the  softer  and 
more  destructible  beds  have  wasted  away  by  atmospheric  and  torrential  action.  Such 
dikes  are  records  of  the  former  existence  of  masses  of  rocks  now  no  more,  though  we 
can  still  trace  the  exact  shape  of  the  fissures  by  which  they  were  at  one  period  traversed. 
5 II  2 
