WHICH  HAVE  CONSOLIDATED  ON  STEEP  SLOPES. 
737 
found  a dry  gulley,  evidently  excavated  by  water,  30  feet  deep,  in  which  many  lavas 
were  intersected  having  irregular  dips,  as  if,  when  they  flowed  into  this  hollow,  the  shape 
of  the  ground  had  been  repeatedly  altered  by  showers  of  sand  and  lapilli,  and  some- 
times by  aqueous  erosion.  Occasionally  the  angle  of  dip  amounted  to  20°  or  even  28°. 
The  stony  beds  scarcely  ever  exceeded  2^  feet  in  thickness,  but  were  often  very  com- 
pact. All  these  are  of  unknown  date,  but  they  cannot  be  of  high  antiquity,  since  on 
this  southern  slope,  where  eruptions  have  been  so  frequent  in  historical  times,  they  con- 
stitute the  outermost  integuments  of  Etna, 
If  it  did  not  appear  to  me  superfluous,  I could  enumerate  other  instances  of  stony 
layers  which  have  congealed  on  slopes  of  from  10°  to  15°,  and  even  higher  angles  in 
other  parts  of  the  great  volcano ; but  the  cases  already  cited  will  establish,  as  a rule,  the 
fact  that  lava  is  capable  of  solidifying  and  forming  stony  and  continuous  layers  on  slopes 
even  steeper  than  those  on  which  loose  cinders  and  lapilli  can  settle.  These  last  may 
accumulate  at  an  angle  of  40°,  or  even  more,  for  I have  seen  scoriae  settle  on  a slope  of 
42°  before  my  eyes  in  the  recent  cone  built  up  in  October  and  November  1857,  within 
the  crater  of  Vesuvius;  but  in  this  instance  the  lava  was  in  a state  of  semi-fusion  when 
it  fell,  and  the  separate  fragments  may  have  been  soldered  together. 
Recapitulation. 
I will  now  briefly  sum  up  some  of  the  leading  conclusions  to  which  we  have  been  led 
by  the  facts  described  in  the  foregoing  pages,  or  first  part  of  this  memoir. 
1st.  Lavas  which  consolidate  on  steep  slopes  at  angles  varying  from  15°  to  40°,  do  not 
consist  of  a confused  mass  of  scoriae  or  of  fragmentary  matter,  but  of  distinct  parts, 
namely,  an  upper  and  lower  mass  of  scoriae,  with  an  intermediate  stony  layer. 
2ndly.  The  core  or  central  portion  forms  a tabular  and  continuous  sheet  of  compact 
stone  parallel  to  the  overlying  and  underlying  scoriaceous  formations,  and  usually  pass- 
ing somewhat  abruptly  into  them. 
3rdly.  The  lower  mass  of  scoriae,  where  the  slope  is  very  steep,  is  more  commonly 
divided  into  distinct  strata  than  the  upper  crust. 
4thly.  There  is  usually  a greater  evenness  and  parallelism  of  the  beds  where  lavas  have 
congealed  at  high  angles,  than  where  they  have  consolidated  on  more  gently  sloping 
ground. 
5thly.  When  successive  streams  of  lava  have  flowed  one  over  the  other  down  steep 
slopes,  the  line  of  junction  between  the  lower  scoriae  of  one  current  and  the  upper  crust 
of  the  antecedent  one  is  often  obliterated. 
5 D 2 
