708 
SIR  CHARLES  LTELL  OX  THE  STRUCTURE  OF  LAVAS 
“ Crater  of  Elevation.”  These  explorations  convinced  me  more  than  ever  of  the  unte- 
nable nature  of  the  hypothesis  which  attributes  so  preponderating  an  influence  in  the 
formation  of  volcanic  cones  to  movements  from  below,  whether  paroxysmal  or  gradual, 
whether  concomitant  with,  or  posterior  to,  the  accumulation  of  the  successive  products 
of  eruption"*. 
In  the  course  of  my  examination  of  Madeira  and  Palma,  I had  seen  modem  lavas, 
inclined  at  high  angles  from  15°  to  20°,  which  had  evidently  not  changed  theh  position 
from  the  time  of  their  origin,  and  which  nevertheless  were  in  great  part  of  stony  struc- 
ture. Mr.  Hartung,  during  a visit  to  Lancerote,  one  of  the  Canaries,  in  1855.  obseiwed 
a basaltic  lava,  compact  and  continuous,  which  had  congealed  on  the  side  of  a modem 
cone  called  the  Corona,  at  an  angle  of  30°,  and  which  he  traced  for  a space  of  20  feet 
down  the  steep  slope,  the  thickness  of  the  layer  increasing  from  2 to  4 feet,  vrith  a 
breadth  of  about  35  feet.  This  current  was  not  more  vesicular  than  some  of  the  oldest 
lavas  of  Madeira  or  of  Etna. 
Mr.  Dana  also,  in  his  ‘ Geology  of  the  Exploring  Expedition  of  the  United  States  ’ 
(1849),  had  remarked  that  the  lava  of  Mount  Loa,  one  of  the  Sanduich  Islands,  cools 
so  suddenly,  as  to  be  capable  of  consolidating  on  steep  slopes,  sometimes  of  25°. 
But  no  observation  was  so  much  in  point,  in  reference  to  this  question,  as  that  made 
by  Signor  Scacchi,  who  in  1850  saw  a compact  stony  lava,  which  in  that  same  year  had 
flowed  down  the  flanks  of  Vesuvius,  from  near  the  margin  of  the  great  crater  to  the 
base  of  the  cone  in  the  Atrio  del  Cavallo,  having  a thickness  of  from  14-  in  the  upper  to 
4^  feet  in  its  lower  part,  and  dipping  at  angles  varying  from  32°  to  38°.  The  interior 
of  this  current  was  laid  open  to  view  by  a rare  accident,  namely,  the  sinkmg  down,  in 
February  1850,  of  a certain  portion  of  the  north  flank  of  the  cone,  whereby  one  side  of 
the  new  lava-stream  was  engulfed,  and  a section  of  the  interior  of  the  remainder  ren- 
dered visible.  This  lava  produced  scarcely  any  scorise,  whether  above  or  below  it,  and 
exliibited  hardly  any  vesicles  in  its  texture.  Although  it  had  cooled  on  an  average 
decliAty  of  35°,  there  was  no  distinction,  in  regard  to  compactness  of  texture,  between 
it  and  those  lavas  of  known  date  at  the  foot  of  Vesmius  that  have  congealed  on  per- 
fectly level  ground 
* For  observations  on  the  structure  of  Palma,  made  by  Mr.  Hartung  and  myself  in  ISoS-oI,  and  theo- 
retical conclusions  thereon,  see  p.  498  et  seq.,  of  Ltell’s  ‘Manual  of  Geology,’  5th  edition,  1855. 
t Scacchi,  Memoria  sull’  incendio  Vesuviano  del  1850  et  1855,  pp.  44,  65,  and  145. 
[Professor  Piazzi  Smtth  informs  me  that  in  1856  he  observed,  within  1700  feet  of  the  summit  of  the 
Peak  of  Tenerilfe,  at  the  Alta  Vista,  a point  10,500  feet  above  the  level  of  the  sea,  a bed  of  dai’k  green 
obsidian  with  crystals  of  glassy  felspar,  resting  on  an  average  slope  of  28°.  The  thickness  of  the  inclined 
lava  (the  interior  of  which  has  been  laid  open  partly  by  aqueous  action  and  partly  by  fractui’es  assisted  by 
gravity)  is  from  3 to  7 feet.  The  rock  is  dense  and  compact  in  its  lower  part,  but  vesicular  towards  the 
surface,  from  which  a covering  of  pumiceous  scoriae  appears  to  have  been  washed  away.  The  width  of  the 
lava  is  200  feet,  and  it  was  traced  for  about  250  feet  down  the  slope  of  tlie  cone.  Dips  of  15°  and  25°  were 
seen  in  the  solid  continuous  layer,  which  is  very  modern  wdth  reference  to  the  Peak  and  the  uppermost  of 
all  the  lavas  on  the  ridge  where  it  occurs  ; but  powerful  streams  of  stdl  later  date,  having  an  aggregate  depth 
