WHICH  HAVE  CONSOLIDATED  ON  STEEP  SLOPES. 
715 
might,  when  the  rocks  helow  were  undermined,  have  slid  down  and  formed  a slope  of  from 
20°  to  30°,  as  from  d to  7,  fig.  3.  The  Bastione  lava,  after  flowing  over  the  platform  from  G 
to  F and  baking  the  underlying  tuff  at  ^,  o,  cZ,  which  it  turned  red,  descended  in  a cascade 
to  the  foot  of  the  old  clifT  D ; but  the  sea  immediately  resuming  its  efforts  cut  away  the 
new  facing  of  lava  B,  C,  F,  and  then  encroached  some  yards  further  into  the  older  rocks 
from  D to  E,  in  the  same  manner  as  it  had  pre’siously  swept  away  the  lavas  A,  B,  C, 
which  formed  the  original  eastward  prolongation  of  Recupeeo’s  seven  currents.  After 
these  complicated  operations,  the  Bastione  lava  itself  was  made  to  end  abruptly  (as  at  o, 
fig.  3)  in  the  face  of  the  actual  precipice  E,  F.  In  the  Canary  Islands,  especially  on  the 
south-western  coast  of  Palma,  I have  seen  numerous  lava-streams  of  modern  date  pour- 
ing in  like  manner  in  black  sheets  over  the  face  of  steep  sea-cliffs,  at  the  base  of  which 
the  waves  of  the  Atlantic  are  beating.  So  many  years  commonly  elapse  between  two 
successive  eruptions,  and  so  many  more  before  a second  stream  of  lava  happens  to  reach 
precisely  the  same  point  of  the  coast,  that  the  sea  has  usually  time  to  remove  a part  of, 
or  all  of  the  new  facing  of  stone  which  for  a season  had  protected  the  old  cliff.  Such 
inroads,  however,  of  the  ocean,  though  they  check  the  advance  of  the  land,  cannot  pre- 
vent the  cliffs  from  gaining  continually  in  height. 
cs[By  referring  again  to  fig  I (p.  712),  the  reader  will  see  at  and  below  C a representa- 
tion of  a great  current  of  lava,  to  which  my  attention  was  first  drawn  by  Dr.  Caelo 
Gemmellaeo,  whose  interpretation  of  its  position  relatively  to  the  ancient  cliff  of  Aci  I 
now  am  prepared  to  adopt  after  examining  it  in  1858.  The  lava  (C)  came  down 
from  the  west  or  from  the  higher  region  of  lateral  cones  over  the  platform  of  Aci,  and 
its  right  margin  is  to  be  seen  in  the  northern  suburbs  of  the  town  at  the  church  of 
Indirizzo  (fig.  I),  where  its  exterior  or  scoriaceous  covering  alone  is  visible.  The  more 
stony  portions  of  the  crust  consist  of  a dark  rock  with  crystals  of  felspar.  The  current, 
when  it  reaches  the  brow  of  the  cliff,  may  be  traced  in  gardens  below  C,  descending  a 
steep  slope  at  angles  of  23°  and  28°,  yet  here  and  there  houses  are  built  on  ledges  of  the 
rock.  The  more  ancient  parts  of  the  cliff  occurring  south  and  north  of  the  course  of  this 
current,  whether  on  the  Aci  or  Santa  Tecla  side,  have  a declivity  of  35°  and  in  some 
places  47°,  being  on  the  average  20°  steeper  than  the  newer  lava.  Thus,  when  viewed  in 
profile  from  the  south,  the  old  cliff  is  represented  by  the  lines  a,  b and  c,  d (fig.  4), 
while  the  new  lava  forms  the  slope  e,  f.  But  no  section  of  the  interior  of  the  mass  e,  f, 
much  less  of  its  junction  with  the  face  of  the  old  buried  cliff,  is  obtainable.  Half-way 
down  from  e to  fl  found  the  same  character  in  the  lava,  as  above,  at  e.  Near  the  sea, 
