WHICH  HAVE  CONSOLIDATED  ON  STEEP  SLOPES. 
757 
right  angles  to  their  dip,  both  m the  Serra  Giannicola  and  Serra  del  Solfizio,  we  soon 
discover  that  we  are  wholly  in  want  of  data  to  test  the  point  at  issue.  We  have  no 
means  of  ascertaining  whether  any  single  stratum  is  persistent  for  an  indefinite  distance 
in  a direction  at  right  angles  to  the  original  com’se  of  the  lava.  The  sections  afforded 
by  the  Serra  Giannicola  and  by  the  rocks  under  the  Montagnuola  (Cuvigghiuni,  Serra 
Intennedia  and  Vavalaci),  are  all  too  interrupted  to  allow  us  to  follow  any  single  bed  of 
rock  very  far  m its  line  of  strike.  Even  in  the  Serra  del  Solfizio  the  gaps  before  alluded 
to  impede  observation,  and  we  have  said  enough  to  show  that  the  alleged  continuity  and 
parallehsm  of  the  beds  in  that  range  of  chff  is  a delusion.  If,  on  the  other  hand,  we 
seek  information  as  to  the  average  breadth,  depth,  and  dip  of  lavas  on  the  slopes  of  any 
modern  cone,  whether  near  the  lip  of  the  crater  or  half-way  down,  or  near  the  base  of 
the  mountain,  we  find  ourselves  still  more  in  the  dark.  In  order  to  appreciate  the  truth 
of  this  remark,  we  have  only  to  consult  the  excellent  monograph  of  Vesuvius  recently 
published  by  Dr.  Roth  of  Berlin,  in  which  he  has  given  us  a judicious  and  elaborate 
analysis  of  the  multitudinous  memou’s  and  treatises  on  the  eruptions  of  that  volcano 
since  the  year  79.  In  consequence  partly  of  the  want  of  sections  on  the  flank  of  a 
growing  cone,  and  partly  of  inattention  to  facts  supposed  to  be  of  no  theoretical  interest, 
we  fail,  in  spite  of  the  high  scientific  attainments  of  so  many  of  the  observers,  to  obtain 
in  any  one  instance  the  three  data  of  which  we  stand  in  need.  Some  record  the  thick- 
ness, others  the  width,  others  the  dip  of  a given  lava-current,  or  the  slope  down  which 
it  flowed ; some  few  mention  two  of  these  conditions,  but  no  one  all  three ; still  less  do 
they  record  facts  respecting  variations  in  breadth,  depth,  and  inclination  at  various 
points  between  the  summit  and  base  of  the  cone*. 
In  S.  V.  Waltekshausex’s  great  map  of  Etna,  we  perceive  that  certain  currents, 
radiating  in  various  directions  from  the  highest  crater  and  platform  (see  Map,  Plates 
XLIX.  and  L.),  have  the  appearance  of  narrow  stripes,  yet  prove,  when  measured  at 
right  angles  to  their  course,  to  have  a width  of  from  300  to  1800  feet.  We  cannot 
ascertain  their  average  depth,  but,  judging  by  their  external  form,  they  probably  exceed 
in  thickness  most  of  the  old  lavas  intersected  in  the  Val  del  Bove.  Nevertheless  they 
* [Since  writing  the  above,  I have  found  some  valuable  observations  on  this  subject,  published  by  Mr, 
Julius  Schmidt  of  Olmutz,  on  the  lavas  of  1855,  ‘Eruption  des  Vesuv.  im  Mai  1855,’  p.  56.  I have  also 
myself  revisited  (September  1858)  Vesuvius  and  the  Atrio  del  CavaUo,  in  company  with  Signor  Gtuiscaedi 
of  Naples.  We  measured  the  depth  and  width  of  several  lavas  of  1857,  resting  on  slopes  of  18°,  24°,  and 
28°,  and  we  compared  the  results  with  those  supplied  by  the  older  lavas  in  the  escarpments  of  Somma,  in 
order  to  see  if  any  of  the  latter  were  of  greater  width  when  intersected  transversely  to  their  dip.  After 
surveying  many  hundreds  of  layers,  we  found  only  one,  near  the  entrance  of  the  Atrio,  on  the  side  of  the 
Observatory,  decidedly  more  persistent  than  the  modem  lavas  examined  by  us.  There  are  beds  having  the 
appearance  of  hard  stony  layers  which  are  conspicuous  in  the  walls  of  the  Atrio,  and  which  might  easily  be 
mistaken  for  lavas,  but  which  consist  of  solid  tuffs.  The  exception  which  we  saw  might,  we  thought,  be 
accounted  for  by  some  accidental  irregularity  in  the  external  slope  of  the  old  cone,  producing  a gibbosity 
such  as  has  been  formed  in  the  last  few  years  on  the  northern  side  of  the  modem  Vesuvius,  and  which,  bv 
intermpting  the  free  course  of  a descending  current,  might  cause  it  to  spread  out  laterally.] 
