WHICH  HAVE  CONSOLIDATED  ON  STEEP  SLOPES. 
731 
by  the  lava  of  1852-53.  Fortunately  at  several  points  the  superficial  scorise,  about  3 feet 
in  thickness,  has  been  washed  ofi"  from  both  of  these  lavas  by  the  rain.  In  the  case  of 
that  of  1852,  we  see  exposed  to  view  the  surface  of  an  underlying  stony  layer,  cellular 
in  part,  or  somewhat  vesicular,  but  continuous,  and  containing  crystals  of  felspar  and 
some  imperfect  ones  of  augite,  with  deep-green  olivine  and  great  abundance  of  titani- 
ferous  iron.  From  the  stony  character  of  the  surface  of  this  bed  immediately  under 
the  scoriaceous  crust,  there  can  be  no  doubt  that  6 or  8 inches  lower  down  it  would 
present  a compact  texture.  The  surface  of  the  continuous  stony  mass  alluded  to  is 
inclined  at  angles  of  35°,  40°,  and  45°,  and  in  one  spot  at  49°,  if  not  50°,  an  angle  which 
my  companion  obtained  with  the  clinometer,  but  not  ■without  some  risk  of  slipping  and 
being  precipitated  to  the  bottom  of  the  precipice.  What  we  had  seen  in  the  Cava  Grande 
(p.  717)  had  prepared  us  to  expect  the  existence  of  such  a tabular  layer  of  stone 
beneath  the  external  scoriee*. 
It  ■will  be  seen  by  the  sketch  (fig.  10)  that  part  of  the  lava  of  1852—53,  after  going 
do’wn  the  precipice,  continued  (at  c,  c)  its  course  along  the  bottom  of  the  nearly  level 
valley,  where  the  slope  is  not  more  than  5°  or  6°.  Here  the  general  depth  of  the 
modern  current  is  e’vidently  greater  than  on  the  slope,  amounting  perhaps  to  8 feet. 
At  one  point  on  the  declmty  of  the  “ Salto”  we  examined  a portion  of  the  lava  of 
1819,  where  it  had  a ■width  of  16  feet,  and  where  the  remainder  or  lateral  prolongation 
of  the  same  was  hidden  by  the  lava  of  1852.  The  older  current  passed  under  the 
newer  ■with  a corresponding  inclination  of  more  than  40°.  In  such  cases  a vertical 
section  would  exhibit  several  parallel  beds,  one  over  the  other,  consisting  alternately  of 
scoriaceous  and  stony  lava.  We  estimated  the  thickness  of  the  lava  of  1852-53,  on 
some  parts  of  the  decli^vdty  (judging  by  its  lateral  walls),  at  about  5^  feet,  of  which  2 or 
2^  may  be  composed  of  stony  matter.  One  of  the  narrowest  streams  or  ramifications 
was  7 0 feet  wide ; so  that  if  the  stony  portion  was  seen  in  a transverse  section,  say  2 feet 
thick,  it  would  appear  as  a thin  sheet  of  matter,  the  horizontal  being  to  the  vertical 
extension  as  30  to  1.  Even  if  the  solid  layer  be  twice  as  thick,  the  width  remaining  the 
same,  it  would  still  constitute  a tabular  mass,  ha^vdng  nearly  even  planes  of  stratification, 
and  these  planes  being  parallel  to  the  upper  and  lower  scoriae. 
Some  geologists  imagine  that  the  general  absence  in  the  walls  of  the  Val  del  Bove  of 
extremely  irregular  surfaces  similar  to  those  of  recent  lava-currents,  or  exhibiting  ridges 
hke  those  of  figs.  6 and  9,  pages  718  and  726,  furnishes  an  argument  against  the  analogy 
of  ancient  and  modern  volcanic  formations.  But  there  are  many  reasons  why  such 
uneven  lines  of  separation  should  be  ra.re  and  exceptional  on  steep  slopes,  and  should 
even  be  obliterated  where  they  have  once  existed. 
In  the  first  place,  if  the  older  lavas  alluded  to  were  formed,  as  I believe  them  to  have 
been,  at  high  inclinations,  their  surfaces  could  not  originally  be  very  uneven,  as  we  learn 
from  the  examples  of  this  “Salto”  at  Calanna  and  of  the  Cava  Grande,  p.  15,  and 
* [After  re-examining  the  “ Salto”  in  1858,  I have  little  to  add  to  the  above  statement.  I myself  found 
a dip  of  48°  in  the  exterior  of  the  cascading  lava  of  1852,  not  far  from  the  north-east  side,  or  Hill  of  Calanna.] 
