742 
SIE  CHAELES  LTELL  ON  THE  STEECTTEE  OE  LAVAS 
corresponding  unconformability  of  the  formations  in  the  clitfs  below  the  Montagnnola 
(in  the  rocks  called  by  S.  v.  Walteeshausev  Vavalaci,  Intermedia  and  Cuvigghiuni), 
the  convergence  of  countless  doleritic  and  other  dikes  towards  the  axis  of  Mongibello, 
as  well  as  of  the  thirteen  or  more  greenstone  dikes  radiating  to  the  centre  of  Tri- 
foglietto, — all  become  explicable  as  soon  as  we  admit  the  theory  of  a double  axis.  We 
may  assume  the  existence  at  an  early  period  of  two  permanent  centres  of  eruption 
(whether  contemporaneous,  like  Kilauea  and  the  summit  crater  of  Mount  Loa  in 
Owhyhee,  or  successive,  hke  Somma  and  Vesuvius),  and  at  a later  period  the  complete 
ascendency  of  what  is  now  the  principal  focus,  that  of  Mongibello,  which  continues  in 
full  vigour,  while  that  of  Trifoglietto  has  long  been  spent.  The  latter  may  have  been 
always  a subordinate  vent,  communicating  at  a great  depth  with  the  main  chimney, 
which  may  never  have  materially  shifted  its  position  from  the  first. 
It  would  clearly  be  unsafe  to  assume,  because  the  upper  part  of  Mongibello  is  newer 
than  the  entire  cone  of  Trifoglietto,  that  therefore  the  latter  is  the  more  ancient  volcano 
of  the  two.  What  is  now  the  great  cone  may  have  attained  half  or  two-thirds  of  its 
actual  height(from  A to  c,  fig.  15,  p.  740),  and  may  possibly  have  been  a ti'achytic  mountain 
before  the  focus  of  Trifoglietto  came  into  play.  The  magnitude  and  volume  of  what  is 
now  the  principal  cone,  having  its  centre  situated  at  the  distance  of  three  miles  from  the 
axis  of  Trifoglietto  (and  therefore  rather  more  than  “un  peu  a Test”),  imply  that  at 
that  point  (the  axis  of  Mongibello),  throughout  longer  periods  of  time  than  anywhere 
else,  the  expansive  gases  and  rivers  of  melted  stone  have  found  their  freest  and  most 
copious  discharge.  But  as  we  have  no  sections  to  enable  us  to  determine  the  relative 
antiquity  of  the  two  foci,  it  would  be  unprofitable  to  enter  more  at  length  into  a discus- 
sion on  the  point  *. 
[During  my  third  visit  to  Etna  in  1858,  I had  opportunities  of  corroborating  the 
observations  of  S.  v.  Waltekshauseiv  respecting  the  dip  of  the  successive  lavas,  scoriae, 
and  tuffs  in  the  Giannicola  and  in  the  other  great  precipices  under  the  Montagnuola.  For 
this  purpose  I made  two  descents,  one  after  sleeping  at  the  Casa  Inglese  from  the  margin 
of  the  Piano  del  Lago  to  the  base  of  the  Giannicola,  and  another  fi:om  the  Montagnuola, 
by  Cuvigghiuni,  Rocca  Intermedia  and  Vavalaci  to  the  base  of  the  same  cliffs  near  the 
Rocca  del  Corvo.  The  steep  inward  inclination  of  the  older  or  inferior  beds  near  the  foot 
of  each  great  chff  is  precisely  as  described  by  S.  v.  WALTEKSHAUSEisr,  and  just  the  reverse 
of  the  direction  we  should  have  anticipated  had  the  highest  part  of  the  mountain  or  the 
axis  of  Mongibello  been  a great  centre  of  upheaval.  Midway  in  one  of  these  lofty 
cliffs  (in  the  Teatro  Grande)  I met  with  a mass  perfectly  horizontal,  consisting  of  compact 
rock,  40  feet  thick,  with  vertical  columns,  yet  having  an  upper  crust  of  scoriae  as  well  as 
a lower  scoriaceous  substratum,  in  a word,  having  all  the  characters  of  a modern  current 
like  that  of  1669  at  Catania,  and  like  it  resting  on  a red  band  of  burnt  tuff.  Nothing 
* [Since  ttie  above  was  written,  I have  received  (February  1859)  Part  VII.  of  S.  vox  WAiiTEESHAtrsEx’s 
‘ Atlas,’  in  which  I observe  the  cone  of  Trifoglietto  (or  Gr  of  pi.  24)  is  supposed  to  be  the  oldest  pai’t  of  Etna. 
It  is  certainly  the  oldest  visible  part,  but  I see  no  reason  for  retracting  the  views  above  set  forth.] 
