WHICH  HAVE  CONSOLIDATED  ON  STEEP  SLOPES. 
729 
before  I had  witnessed  the  eruption  of  Vesuvius  in  1858,  and  had  seen  a ridge  in  the 
act  of  forming,  I speculated  on  the  possibility  of  the  lava  of  1852  having  been  partially 
forced  into  its  present  shape  hy  lateral  pressure,  caused  by  the  successive  flows  of  lava 
having  been  piled  one  over  the  other,  while  the  interior  of  some  of  the  currents  first 
poured  out  was  still  viscous  or  even  liquid.  The  great  weight  and  thickness  of  the  new 
flows  might,  I imagined,  have  given  rise,  by  hydrostatic  pressure,  to  effects  like  those 
witnessed  where  certain  railway  embankments  have  been  thrown  across  marshes  and  peat 
mosses.  In  such  cases,  it  is  frequently  found  that  as  fast  as  new  matter  is  cast  upon  them 
the  mounds  sink  down  bodily,  while  on  one  or  both  sides  the  surface  of  the  bog  or 
morass  swells  up  in  one  or  more  parallel  undulations.  I also  referred  to  the  analogous 
efiects  of  downward  pressure,  to  which  my  attention  had  been  directed  by  Dr.  Gould  in 
1852  at  Boston  in  the  United  States,  where  a load  of  sand  and  stones,  upwards  of 
900,000  cubic  feet  in  volume,  had  been  thrown  into  part  of  an  estuary  only  dry  at  low 
water  in  order  to  convert  it  permanently  into  land.  In  consequence  of  the  pressure, 
the  adjoining  bottom  of  the  estuary,  supporting  a dense  growth  of  salt-water  plants,  and 
previously  only  just  visible  at  low  tide,  was  pushed  gradually  upward,  in  the  course  of 
many  months,  so  as  to  project  5 or  6 feet  above  high-water  mark.  The  upraised  mass 
was  bent  into  five  or  six  anticlinal  folds,  and  the  upper  layer  of  turf  having  burst  open 
along  the  crests  of  the  ridges,  exposed  to  view  an  underlying  layer  of  mud  full  of  recent 
marine  shells*.  But  I abandoned  this  explanation  of  the  origin  of  the  Etnean  ridges 
after  I had  seen  similar,  though  much  smaller  ones,  produced  on  the  flanks  of  Vesu- 
vius in  the  manner  about  to  be  described. 
When  I visited  that  volcano  in  September  1858,  I found  that  the  eruption  of  the 
preceding  spring  had  not  wholly  ceased.  Intermittent  jets  of  vapour,  illumined  by  the 
hot  lava  of  the  crater  below,  were  still  issuing  from  the  summit,  while  two  minor  cones 
at  the  western  foot  of  the  principal  cone,  and  just  below  the  Observatory,  were  pouring 
out  continually  small  rills  of  lav'a  unaccompanied  by  any  evolution  of  gases.  Some  of 
these  rills  ran  very  rapidly  near  then*  sources,  but  when  they  reached  the  base  of  the 
cone  proceeded  with  extreme  slowness.  Here  I watched  the  progress  of  one  of  them, 
about  8 feet  high,  having  the  form  of  a steep-sided  and  narrow  ridge,  its  sharp  crest 
crovvTied  with  just  such  irregular  and  grotesque-shaped  fragments  of  rock  as  are  seen  on 
the  new  lavas  of  the  Val  del  Bove.  Its  motion  was  onwards  in  a straight  line,  but  was 
only  appreciable  by  great  attention.  The  change  of  position  of  the  protruding  frag- 
ments on  the  crest  was  only  verified  by  obseiwing  their  altered  relation  to  other  fixed 
and  motionless  points  of  lava  just  beyond.  Viewed  in  the  daytime,  the  advancing 
ridge  was  black,  but  now  and  then  one  of  the  steep  sides  was  seen  to  swell  out  as  if 
composed  of  viscous  matter,  and  then  to  crack,  disclosing  a glowing  and  white  heat 
within.  This  opening  was  soon  followed  by  an  avalanche  of  fragments,  black  on  one 
side  and  red-hot  on  the  other,  rolling  down  with  a clattering  sound  to  the  bottom  of  the 
slope.  Instead  of  an  escape  of  lava  and  levelling  up  of  the  ground  on  one  side,  or  any 
* See  Ltell’s  ‘ Manual  of  G-eologj,’  p.  136,  5th  edit. 
5c2 
