736 
SIE  CHAELES  LTELL  ON  THE  STErCTHEE  OF  LAVAS 
scorige  of  each  should  touch  and  join  in  the  middle  of  the  section,  then  h would  seem  to 
be  a prolongation  of  a with  merely  one  of  those  slight  breaks  or  thinnings  out  and 
resumptions  of  beds  with  which  we  are  familiar. 
I may  observe  in  concluding  this  subject,  that  if  there  be  cases  (though  I did  not 
happen  to  see  one  on  Mount  Etna)  where  the  melted  matter  has  run  out  from  the  inte- 
rior of  a stream  of  lava  which  has  descended  a steep  slope,  so  as  to  cause  the  superficial 
crust,  together  with  the  solid  central  layer,  to  fall  and  cover  in  separate  pieces  the  floor 
of  the  arched  tunnel,  this  fact  would  not  favour  the  notion  of  those  who  question  the 
analogy  of  ancient  and  modem  formations  in  volcanic  mountains.  The  result  of  the 
collapse  would  be  simply  this,  that  a stratum  of  fragmentary  and  scoriaceous  materials 
of  unusual  thickness  would  be  seen  in  the  area  of  the  downfall. 
Highly  inclined  modern  lava  near  the  Montagnuola. 
In  the  Map  of  the  Val  del  Bove  (Plate  L.),  the  position  of  the  cone  called  the  Mon- 
tagnuola will  be  seen.  The  summit  of  the  steep  escarpment  immediately  below  and  to 
the  east  of  it  is  called  the  Schiena  del  Asino.  In  order  to  pass  from  the  Val  del  Bove  to 
the  upper  region  of  Etna,  we  ascended  this  precipice,  more  than  2Q00  feet  high,  observmg 
the  edges  of  a vast  series  of  ancient  volcanic  rocks,  some  crystalline  but  mostly  fragment- 
ary, which  dipped  to  the  south-west  or  inwards,  and  away  from  the  Val  del  Bove.  At 
some  period  since  the  formation  of  this  great  escarpment,  and  consequently  since  the 
origin  of  the  Val  del  Bove  (of  which  it  forms  the  southern  boundary),  a lava  of  unknown 
date,  but  evidently  from  its  external  characters  not  very  ancient,  has  poured  over  the 
brow  or  upper  margin  of  the  cliff,  and  running  down  the  slope  has  covered  the  outcrop- 
ping edges  of  the  older  lavas  and  scoriae.  As  so  often  happens  where  the  slope  is  great, 
the  interior  of  this  current  is  laid  open  at  its  side  by  the  waste,  from  atmospheric  causes, 
of  its  scoriaceous  crust.  Its  thickness  is  20  feet  where  it  commences  its  descent,  on 
ground  inclined  at  an  angle  of  30°,  and  where  the  declivity  augments  to  about  35°,  the 
thickness  diminishes  to  about  15  feet. 
Here  as  elsewhere,  there  is  an  upper  and  a lower  scoriaceous  mass,  but  somewhat 
more  than  half  of  the  whole  consists  of  stony  lava,  more  or  less  vesicular'.  We  have  in 
this  case  therefore  a solid  layer,  varying  from  8 to  more  than  10  feet  in  depth,  the 
thickest  seen  by  us  at  so  high  an  angle.  The  current,  after  flowing  down  a few  hun- 
dred yards,  seems  to  have  been  exhausted,  and  is  for  the  last  few  yards  of  its  comse 
broken  up  into  detached  and  more  or  less  scoriaceous  fragments. 
Leaving  the  Val  del  Bove,  we  then  examined  some  sections  afforded  by  the  charmels 
of  torrents,  dry  at  the  end  of  October,  which  here  and  there  have  eaten  into  the  flank 
of  the  great  cone  of  Etna,  between  the  Schiena  del  Asino  and  the  town  of  Nicolosi.  Vs  e 
had  not  gone  far  from  the  “ Casa  del  Vescovo”  (sometimes  called  the  Casa  delle  Neri), 
before  we  saw  in  one  of  the  watercourses  alluded  to,  a lava  dipping  at  various  angles 
between  26°  and  29°,  and  having  a thickness  of  between  5 and  6 feet.  As  usual,  there  was 
a thin  stony  layer  forming  the  core  or  central  part.  A mile  or  more  fm'ther  doA^•n  we 
