WHICH  HAVE  CONSOLIDATED  ON  STEEP  SLOPES. 
719 
large  part  of  the  old  rocks  {e,f)  in  the  cliffs  of  the  right  and  left  banks.  The  joints  are 
few,  often  9 or  10  feet  apart,  so  that  detached  fragments  of  this  stone,  9 feet  in  length 
and  5 feet  high,  may  be  seen  strewed  over  the  slope  on  the  left  side  of  c,  d,  fig.  6.  The 
demarcation  between  the  stony  layer  and  bottom  scorise  is  marked,  whereas  the  passage 
of  the  same  into  the  upper  scorise  is  more  gradual.  [The  normal  thickness  of  the  solid 
rock  is  5 feet,  where  the  inclination  is  32°  and  35°;  but  when  near  the  top,  as  at  e\  the 
dip  increases  to  45°  and  47°,  the  thickness  is  reduced  to  one-half,  or  2^  feet,  just  as  a 
stream  of  water,  arrmng  at  a steep  part  of  its  channel,  increases  in  speed  and  diminishes 
in  depth.  Yet,  when  dipping  at  47°,  it  is  still  not  only  stony  and  compact,  but  there  is 
no  breach  whatever  of  continuity  in  the  mass,  and  no  more  joints  than  in  the  less 
inclined  portion. 
The  first  operation  which  took  place  when  the  lava  reached  the  edge  of  the  ravine, 
seems  to  have  been  the  rolling  down  of  scoriae  from  the  frontal  wall  of  the  advancing 
current,  so  that  the  imeven  sides  of  the  ravine  were  levelled,  and  a sloping  talus  of  loose 
scoriae  [g),  for  the  most  part  inclined  at  32°  to  35°,  was  produced.  But  near  the  top,  at 
€\  before  the  old  precipice  had  been  reduced  to  that  more  moderate  inclination,  the  lava 
cooled  at  an  angle  of  47°,  and  might  probably  have  consolidated  on  a still  steeper  slope. 
The  width  of  the  branch  c,  d must  exceed  400  feet,  and  is  therefore  so  great  in  pro- 
portion to  its  depth,  that  in  a transverse  section  the  central  stony  layer  of  5 feet  would 
be  seen  to  form  what  might  be  termed  a sheet  of  lava. 
The  traveller  will  at  once  see,  by  the  state  of  the  surface  of  this  current  and  by  its  vege- 
tation, that  it  is  not  a current  of  high  antiquity.  Except  where  cultivated  or  planted  with 
trees,  it  is  covered  simply  with  lichens  or  with  a few  scattered  shrubs,  chiefly  of  broom. 
The  superficial  in-egularities  of  the  main  current  (a,  c),  when  contrasted  with  the  compa- 
rative evenness  of  the  branch  c,  d,  are  very  striking,  the  first  inclined  at  16°  east,  the  other 
at  32°  and  47°  north.  Indeed,  the  height  of  the  parallel  ridges,  from  a to  no  less  than 
four  of  which  come  within  a few  hundred  paces  of  the  Cava  Grande,  is  excessive,  as  is 
the  steepness  of  their  sides.  The  ridge  h is  about  40  feet  high  and  40  wide,  its  north 
side,  or  that  towards  the  Cava  Grande,  sloping  at  an  angle  of  70°,  its  southern  at  35° 
and  40°;  the  mineral  composition  of  the  concentric  layer  of  scoriaceous  lava  forming  the 
outer  part  of  the  ridge  being  similar  to  that  of  the  branch  c,  d,  fig.  6.  The  continuity 
of  the  branch  c,  d with  the  main  current  (a,  Z»),  is  such  as  to  preclude  the  supposition  of 
the  slightest  change  of  position  in  c,  d since  1689,  when  it  cascaded  into  the  Cava 
Grande ; and  if  there  were  no  other  section  on  Mount  Etna  to  illustrate  the  capability  of 
lava  to  form,  when  cooling  on  a steep  slope,  a dense  and  continuous  layer  of  stone,  such 
as  would  be  termed  trap  in  the  ordinary  language  of  geologists,  this  example  in  the 
Cava  Grande  would  suffice.] 
Structure,  aspeci,  and  inclination  of  the  lavas  of  the  great  eruption  q/‘ 1852-53. 
From  the  Cava  Grande  we  went  by  Milo  to  Zafarana,  and  observed  at  the  former 
place  some  recent  changes  which  the  last  great  eruption  of  Etna,  that  of  1852,  had 
AIDCCCLVIII.  5 B 
