WHICH  HAVE  CONSOLIDATED  ON  STEEP  SLOPES. 
717 
lava,  in  this  case  of  known  date,  conforming  to  a very  steep  slope,  and  having  its  internal 
structure  remarkably  well  displayed.  Our  bridle-road  led  us  round  the  head  of  a deep 
and  narrow  guUey,  called  the  Cava  Grande  (see  Maps,  Plate  XLIX.  and  L.),  the  largest 
of  several  which  furrow  this  part  of  the  eastern  flank  of  Etna.  Looking  down  the 
ra\ine,  we  saw  on  the  right  bank  a stratified  mass,  turned  edgeways  towards  us  (5,  fig.  5), 
Eig.  5. — Highly  mclvaed  lava  of  Cava  Grande. 
a,  a.  Main  stream  of  lava  of  1689. 
b,  Branch  of  same  lava  (c,  d,  fig.  6),  having  a mean  inclination  of  35°. 
c,  c'.  Eight  bank  of  ravine  composed  of  older  lavas. 
which  I thought  at  first  must  be  a portion  of  the  beds  of  lava  and  scoriae  which  had  once 
belonged  to  the  upper  part  of  the  cliff,  and  which,  having  been  undermined  by  the 
torrent,  had  slid  down  the  steep  bank.  But  on  nearer  inspection  we  ascertained  that  the 
inclined  mass  (5,  fig.  5)  was  a branch  of  a,  a,  or  the  great  current  of  1689,  which  flowed 
dowmfrom  the  Val  delBove  in  that  year,  and  cascaded  in  the  manner  represented  in  the 
annexed  drawing  (fig.  5),  over  the  right  bank  of  the  Cava  Grande.  The  ramification 
of  this  current  is  very  accurately  laid  down  in  the  Map  of  S.  von  Walteeshausen,  copied 
in  Plate  L.,  where  the  date  assigned  to  it  is  1688;  but  Dr.  Giuseppe  Gemmellaeo  in 
his  Map  of  Etna  has  given  the  date  as  1689,  and  he  tells  me  that  the  lava  of  the  pre- 
ceding year  was  confined  to  the  Val  del  Bove. 
For  a great  part  of  the  year  there  is  no  water  flowing  in  the  Cava  Grande,  nevertheless 
occasional  floods  have  in  the  course  of  ages  excavated  this  deep  narrow  chasm,  at  the 
upper  end  of  which  we  find,  what  is  so  common  at  the  head  of  narrow  valleys  on  the 
sides  of  volcanos,  a perpendicular  precipice,  of  a horse-shoe  form,  over  which  water  falls 
in  a cascade,  which  is  gradually  cutting  its  way  backwards.  [Although  the  channel  was 
