7G8 
SIE  CHAELES  LYELL  OX  THE  STEHCTUEE  OF  LAVAS 
lip  from  beneath  the  gravel  and  formed  the  steep  sides  of  the  raiine.  In  the  course  of 
time  the  waters  again  excavated  a new  channel,  deeper  than  the  first  one.  but  not  always 
coinciding  with  it  in  all  its  windings,  although  mainly  in  the  same  direction.  This  new 
erosion  passed  through  both  the  basalt  and  subjacent  tufi",  with  or  without  the  inter- 
vention of  the  ancient  river-bed,  according  as  the  new*  water-course  corresponded  with  or 
deviated  from  the  older  one.  Lastly,  the  lava  of  the  year  1284,  called  the  “ Sciara  di 
femina  morta,”  flowed,  as  will  be  seen  by  the  Map,  Plate  XLIX,  from  the  Yal  del  Bove 
west  of  Milo,  and  then  north  of  Ballo  to  La  Macchia,  entering  the  modem  channel  of 
the  torrent  below  that  town,  and  is  now  in  its  turn  cut  through  by  the  torrent,  as  I saw 
when  I revisited  the  section  in  1858.  From  this  last  fact  I infer,  that  as  this  mediaeval 
lava  came  down  to  this  point  from  the  Val  del  Bove,  so  many  of  the  antecedent  floods 
of  melted  matter,  and  among  them  the  basaltic  cun’ent  above  alluded  to,  was  derived 
from  that  region.  They  would  naturally  all  follow  the  low^est  levels  of  the  then  existing 
surface  of  the  country,  just  as  the  waters  draining  the  eastern  slopes  of  Etna  have  done. 
For  this  reason  the  older  lava  filled  the  old  river-bed,  and  afterwards  the  newer  lava  of 
1284  reached  the  existing  river-channel. 
We  might  have  expected  that  what  was  once  an  ancient  allmial  plain  near  the  sea 
should  now  form  a terrace  at  Giarre  40  or  50  feet  above  the  level  of  the  existing  drain- 
age, because  the  whole  region  must  have  participated  in  that  upward  movement,  of 
which  we  find  so  many  proofs  along  the  sea-coast.  During  the  gradual  rise  of  the  land 
the  waters  would  hollow  out  their  new  bed,  through  gravel  and  incumbent  lava,  more 
readily  than  if  the  levels  had  been  stationary.  Upon  the  whole,  I come  to  the  conclu- 
sion that  the  drift  of  Giarre  bears  the  same  relation  to  the  Yal  del  Bove  which  the 
fluviatile  conglomerate  of  the  Barranco  de  las  Angustias  bears  to  the  Caldera  of  Palma 
in  the  Canaries ; that  conglomerate  of  Palma  is  800  feet  thick  and  4 miles  long,  com- 
posed entirely  of  fragments  of  volcanic  rocks,  and  associated,  like  that  of  Giarre,  with 
recent  lavas  of  modern  date 
^[Hoiv  far  successive  subsidences  and  explosions  cooperated  informing  the  Yal  del  Bove. 
But  we  can  by  no  means  attribute  the  origin  of  the  Yal  del  Bove,  any  more  than  the 
Caldera  of  Palma,  exclusLely  to  the  action  of  running  water.  In  the  case  of  the  great 
Etnean  cavity,  the  northern  and  southern  escarpments  are  too  widely  separated  Loin 
each  other,  and  the  western  boundary  cliff,  or  Balzo  del  Trifoglietto  (nearly  4000  feet 
high),  is  too  lofty  to  allow  of  such  an  hypothesis.  Some  one  or  more  local  catastrophes 
of  paroxysmal  intensity  may  have  given  rise  to  the  first  breaches,  which  ended  in  pro- 
ducing this  enormous  cavity,  occupying  about  one-sixth  of  the  circumference  of  the 
great  cone,  a ca\ity  to  which  there  is  nothing  similar  on  the  other  sides  of  the  volcano. 
What,  then,  we  must  now  inquire,  were  the  peculiar  and  exceptional  causes  to  which 
it  owed  its  origin  1 I have  already  spoken  of  the  Cisterna,  an  elliptical  cavity,  now 
about  120  feet  deep,  produced  in  the  year  1792  on  the  platform  of  the  Piano  del  Lago 
* See  ‘ Manual  of  Greology,’  by  the  author,  5th  edit.  p.  507. 
