WHICH  HAVE  CONSOLIDATED  ON  STEEP  SLOPES. 
735 
which  is  50  feet  broad,  so  as  to  undermine  the  incumbent  stony  bed,  and  cause  a vertical, 
or  here  and  there  a more  than  vertical  section,  for  the  stony  layer  occasionally  overhangs 
4 or  5 feet.  The  lower  scoriae  are  divided  into  distinct  strata,  and  contain  masses  of  ropy 
lava  and  volcanic  bombs.  The  solid  bed  (C  2)  is  usually  very  compact,  and  although  in 
parts  vesicular,  is  on  the  whole  more  stony  than  the  average  of  the  older  lavas  of  the 
Val  del  Bove,  or  of  the  precipice  called  Balzo  del  Trifoglietto.  The  rock  consists,  like  so 
many  of  the  modern  lavas  of  Etna,  of  a dark-coloured  base,  in  which  are  many  crystals  of 
felspar  and  some  of  olivine.  Its  specific  gravity  is  2 '785. 
At  the  lower  end  [d),  where  a transverse  section  is  obtained  in  consequence  of  frag- 
ments of  the  terminal  crust  having  fallen  dovm,  we  find  the  whole  thickness  of  the  mass 
reduced  to  8 feet,  the  top  scoriae  (C  1)  being  from  3 to  4 feet  thick,  below  which  is  the 
usual  stony  bed,  here  of  a bluish  colour,  from  1 to  2 feet  thick,  and  then  the  lower 
scoriae.  At  the  same  point  [d),  I observed  from  below  the  current  C another  antecedent 
lava  (B)  with  a powerful  upper  crust  of  scoriae  emerging  with  a like  steep  dip.  There 
may  possibly  be  some  other  conformably  inchned  modern  lavas  between  B and  A,  which 
last  represents  the  nearly  horizontal  older  rocks,  which  in  this  part  of  the  precipice  we 
know  from  numerous  sections  dip  gently  to  the  south  and  south-east. 
For  several  hundred  yards  down  the  face  of  the  precipice  I ascertained  the  continuity 
of  the  current  (C,  fig.  12),  but  at  one  point  I found  that  the  entire  mass  had  been  rent, 
probably  by  an  earthquake,  in  a direction  at  right  angles  to  the  course  of  the  lava,  so 
that  an  open  fissure  2 feet  or  more  wide  now  discloses  its  internal  structure,  showing 
that  the  stony  sheet  of  rock  in  the  centre  is  persistent  in  a transverse  as  well  as  in  a lon- 
gitudinal direction,  afibrding  us  an  opportunity  of  determining  that  it  does  not,  like 
some  narrow  lava-streams  on  a steep  slope,  sink  in  the  middle,  the  centre  being  lower 
than  the  sides,  as  if  the  lava  had  run  out  and  caused  the  crust  to  fall  in.  The  open  rent 
may  imply  the  sliding  down  of  a portion  of  the  mass  at  the  time  of  the  fracture ; in- 
deed I could  not  look  at  the  dip  of  this  massive  current  (C),  especially  its  central  com- 
pact layer,  without  wondering  that  the  whole  has  not,  during  the  reiterated  shocks  to 
which  this  part  of  the  mountain  is  subject,  been  hurled  down  into  the  valley  below.] 
Many  geologists,  on  beholding  from  a distance  several  of  these  narrow  strings  or  rills 
of  molten  matter  congealed  on  the  surface  of  a great  talus  of  sand  and  scoriae  like  that 
below  the  Cisterna,  may  imagine  that  a cross  section  could  never  present  to  them  any 
appearances  analogous  to  those  which  we  witness  in  the  Val  del  Bove  or  in  the  Atrio  del 
Fig.  13. — Appearance  in  a cross  section  of  two  juxtaposed  narrow  streams  of  lava. 
CavaUo.  But  we  must  remember  that  the  lava  above  described  is  50  feet  wide,  and  if 
the  solid  layer  be  6^  feet  thick,  it  would  be  like  a in  the  accompanying  diagram  (fig.  13); 
and  if  another  similar  current  should  flow  down  by  its  side,  so  that  the  outer  and  lateral 
MDCCCLVIII.  5 D 
