732 
SIE  CHAELES  LTELL  OX  THE  STEUCTTEE  OF  LAVAS 
others  hereafter  to  be  cited.  2ndly.  The  bottom  scorise  of  the  newer  current  is  usually 
so  dovetailed  into  the  top  scoriae  of  the  older  which  it  overflows,  and  so  amalgamated 
with  it,  that  the  line  of  junction,  instead  of  being  marked,  will  usually  be  obliterated. 
Let  the  uppermost  line  in  fig.  11  represent  the  uneven  smlace  of  the  most  recent  of  a 
Fig.  11. — Obliteration  of  the  junction-lines  of  successive  lava-streams. 
series  of  lavas  which  have  been  piled  one  over  the  other  on  a slope  of  25°.  The  lava 
C,  the  newest  of  the  whole,  consists  of  three  parts,  namely,  C 1 the  lower  scoriae,  C 2 
the  central  stony  portion,  and  C 3 the  upper  scoriae.  It  null  be  seen  that  the  bed  last 
mentioned  (C  3),  although  extremely  irregular  at  top,  is  much  less  so  at  bottom,  where  it 
passes  somewhat  suddenly  into  the  stony  layer  C 2 ; and  again,  the  latter  passes  still 
more  abruptly  from  the  solid  to  the  scoriaceous  form  mto  C 1,  in  wliich  the  upper  plane 
of  stratification  is  tolerably  even.  As  this  lower  bed  of  scoriae  (C 1)  had  to  adapt  itself 
to  all  the  superficial  inequalities  of  the  antecedent  lava  B 3,  it  might  have  been  expected 
to  afibrd  a junction-line  at  its  base  as  uneven  as  the  dotted  line  y,  g ; but  no  such  line 
occurs  in  nature,  because,  in  the  first  place,  the  two  lavas  B and  C following  each  other 
next  in  order  of  time  are  usually  of  similar  composition,  and  the  bottom  scorite  of  the 
newer  blends  into  a uniform  mass  with  the  upper  scorise  of  the  older  ciurent,  so  that 
we  cannot  discover  (as  between  y,  K)  where  one  ends  and  the  other  begins.  If  the  heat  of 
C 1 be  sufficient  to  reduce  some  part  of  B 3 to  a state  of  fusion  or  semifusion,  it  -^ill  be 
still  more  difficult  to  tell  which  is  which.  If  the  overflowing  lava  moves  rapidly,  it  can 
scarcely  fail  to  exert  a certain  amount  of  friction  on  the  bottom,  so  as  to  abrade  and 
level  down  such  asperities  as  occur  on  the  top  of  C 3,  and  which  originally  belonged  to 
the  surface  of  B 3.  But  if  the  lava  moves  slowly,  as  it  sometimes  does  even  doT^n  a steep 
cone,  then  another  levelling  operation  comes  into  play,  one  which  I \\dtnessed  dming 
the  eruption  of  Vesuvius,  on  October  14, 1857,  in  the  Atrio  del  Cavallo.  The  lava  had 
been  flowing  for  more  than  two  days,  and  had  gone  domi  from  the  margin  of  the  great 
crater  to  its  base,  and  was  there  proceeding  at  a leisurely  pace  over  more  even  ground. 
So  slight  was  the  motion,  that  the  mass  seemed  stationary,  till  after  watching  it  for  a 
