718 
SIE  CHAELES  LTELL  ON  THE  STErCTEEE  OE  LAVAS 
dry  at  the  time  of  my  last  ^isit  (September,  1858),  I witnessed  several  avalanches  of 
sand  and  stones  loosened  by  the  heavy  rains  of  the  preceding  day,  proving  to  me  how 
great  must  have  been  the  waste  of  these  cliffe  in  the  course  of  the  170  years  which  have 
elapsed  since  the  lava  [h)  poured  over  the  right  banh.  The  geologist  is  indebted  to 
such  annual  waste  not  only  for  the  section  of  the  lower  extremity  of  the  lava,  but  also 
for  that  of  its  side,  as  shown  in  fig.  5,  from  which  great  fragments  have  broken  away 
for  a space  of  about  200  feet  in  length,  many  of  them  still  strewed  over  the  slope.] 
The  ravine  is  about  220  feet  deep,  with  walls  in  parts  vertical,  but  whose  mean  inclina- 
tion ranges  from  38°  to  65°,  usually  about  20°  steeper  than  the  branch  of  the  lava  (5) 
of  1689. 
The  entire  thickness  of  the  latter,  so  far  as  it  is  \isible,  is  on  an  average  about  16  feet, 
consisting  of  three  parallel  parts — an  upper  and  scoriaceous  crust  8 feet  thick,  a stony 
and  very  compact  bed  of  rock  5 feet  thick,  and  a regularly  stratified  mass  of  underlying 
scoriae,  of  which  3 feet  and  sometimes  more  are  visible. 
Fig.  6. — Section  in  part  ideal  of  ancient  and  modern  lavas  in  the  Cava  Grande  near  the  head  of  the  ravine. 
b 
a,  c.  Lava  of  1689,  with  lofty,  parallel,  east  and  west  ridges. 
c,  d.  Branch  of  same  current  descending  the  right  bank  of  the  Cava  Grande,  and  inclined  at  35°  and  higher 
angles,  the  dark  central  portion  being  compact,  the  rest  scoriaceous. 
The  annexed  section  (fig.  6)  is  supposed  to  pass  through  the  Cava  Grande  in  a north 
and  south  direction,  or  at  right  angles  to  its  com’se,  and  to  intersect  not  only  the  newer 
lava  of  1689  (a,  c,  d),  but  also  the  nine  or  ten  older  currents  of  Etna  and  e\f') 
which  constituted  the  walls  of  a ravine  before  the  modern  lava  poui’ed  into  it.  The  old 
cuirents  alluded  to  appear  horizontal  in  the  diagram,  because  they  are  intersected  at 
right  angles  to  their  dip.  They  are  in  fact  inclined  at  about  7°  eastward  or  towards  the 
sea,  each  lava  being  on  an  average  about  10  or  12  feet  thick,  and  separated  by  fi'ag- 
mentary  matter  belonging  to  the  top  and  bottom  scoriae  of  successive  currents,  or  some- 
times by  tuff,  ^.  e,  volcanic  sand  and  mud  transported  by  land-floods  or  the  'winds.  These 
partings  are  about  equal  in  thickness  to  the  sohd  beds,  and  exhibit  at  various  heights 
red  or  burnt  strata,  as  at/",  three  of  them  being  conspicuous  on  the  left  bank. 
To  return  to  the  branch  of  the  lava  of  1689,  or  c,  d,  fig.  6 : I have  stated  that  its 
central  portion  is  quite  compact.  It  contains  crystals  of  felspar,  but  none  of  augite,  a 
small  quantity  of  olivine,  no  iron,  and  its  specific  graAity  is  that  of  an  ordinary  trap-rock. 
It  is  worthy  of  notice,  that  this  central  inclined  layer  surpasses  greatly  in  compactness  a 
