WHICH  HAVE  CONSOLIDATED  ON  STEEP  SLOPES. 
741 
between  c and  cZ,  fig.  15),  a space  gradually  filled  up  by  lavas  and  fragmentary  matter, 
the  stratification  of  which  would  be  occasionally  horizontal,  and  always  much  less 
inclined  than  that  formed  where  two  cones  have  not  mutually  interfered  with  each 
other’s  regular  growth.  If,  then,  we  name  one  of  the  two  centres  the  axis  of  Tri- 
foghetto,  as  before  suggested,  we  may  call  the  other,  or  the  present  great  centre  of  acti- 
vity, the  axis  of  Mongibello,  the  modem  Sicilian  appellation  for  Mount  Etna. 
The  late  Maeio  Gemmellako  is  cited  by  M.  E.  de  Beaumont*  as  having  first  sug- 
gested the  idea  “ that  Etna  was  not  a simple  cone  with  a single  axis,  but  was  made  up 
of  two  cones,  one  of  them  composed  of  the  more  ancient  rocks  (namely,  those  seen  in 
the  Val  del  Bove),  the  other  of  more  modern  products.  The  former,  he  observed,  is 
placed  at  a short  distance  to  the  east  (un  peu  a I’est)  of  the  other,  and  is  not  entirely 
embraced  by  the  products  of  the  more  modern  cone.”  The  same  experienced  observer 
(Makio  Gemmellaeo)  first  remarked,  that  in  the  modern  eruptions  of  Etna,  when  lateral 
cones  are  thrown  up  in  a linear  series,  they  radiate  towards  the  present  crater  (or  towards 
the  axis  of  Mongibello),  as  if  the  rending  of  the  mountain  proceeded  from  that  great 
central  focus. 
The  former  existence  of  an  old  centre  of  eruption  in  the  Piano  del  Trifoglietto,  was 
inferred  from  independent  evidence  by  S.  von  WALTEESHAUSENf,  ^.  e.  from  observations 
first  made  by  him  on  the  convergence  towards  a middle  point  in  that  area  of  thirteen  or 
more  dikes  of  greenstone  visible  in  the  surroimding  escarpments,  one  of  them  of  enor- 
mous dimensions,  or  70  feet  in  width.  The  same  geologist,  after  minutely  scrutinizing 
the  structure  of  those  rocks  which  project  like  gigantic  buttresses  from  cliffs  2000  and 
3000  feet  high  between  the  Giannicola  and  the  Rocca  del  Corvo  (including,  therefore, 
the  cliffs  below  the  Montagnuola),  ascertained  that  while  the  beds  dip  at  high  angles 
inwards  or  towards  the  escarpment  {i.  e.  away  from  the  Val  del  Bove),  in  the  lower  half 
of  the  precipices  they  become  horizontal  in  the  middle  portion,  and  towards  the  summit 
dip  as  if  they  were  sloping  away  from  some  other  point  near  the  present  great  centre 
of  Mongibello.  He  found  in  the  rocks  called  by  him  Teatro  Piccolo  and  Teatro  Grande 
(see  Map,  Plate  L.),  above  the  base  of  the  Serra  Giannicola,  where  I noticed  the  steep 
north-west  dip  of  the  trachytic  formations,  a nearly  horizontal  stratification  in  beds  inter- 
sected by  a multitude  of  vertical  dikes. 
Signor  G.  G.  Gemmellaeo  and  I had  an  opportunity  in  1857  of  verifying  these  obser- 
vations, so  far  as  regards  the  steep  south-west  dip  of  the  beds  in  the  lower  half  of  the 
cliffs  below  the  Montagnuola ; and  when  we  looked  down  from  the  top  of  the  precipice, 
called  in  S.  v.  Walteeshausen’s  map  the  Schiena  del  Asino,  we  saw  the  unconformable 
inclination  of  the  superior  doleritic  lavas,  which  is  well  expressed  in  plate  7 of  ‘ The 
Atlas  of  Etna.’  We  were  also  struck  with  the  obvious  convergence  of  a multitude  of 
vertical  dikes,  in  the  precipices  below  the  Montagnuola  and  in  the  Balzo  di  Trifoglietto, 
towards  the  present  centre  or  axis  of  Mongibello.  The  whole  of  these  phenomena, — 
the  change  of  dip  in  the  inferior,  medial,  and  uppermost  beds  of  the  Giannicola,  the 
* Eecherclies  sur  I’Etna,  p.  124.  t Atlas,  V.  and  VI. 
