WHICH  HAVE  CONSOLIDATED  ON  STEEP  SLOPES. 
733 
few  minutes,  we  saw  fragments  detach  themselves  and  roll  down  the  steep-fronted 
terminal  wall,  and  not  stop  till  they  were  far  in  advance  of  the  main  body  of  the  current. 
Such  blocks  would  soon  level  up  any  depression  that  might  exist  in  the  superficial  crust 
of  an  older  lava,  and  would  also  tend  to  assimilate  in  character  the  component  parts  of 
the  upper  and  lower  scoriae.  Passages  of  this  kind  of  the  top  of  one  lava-stream  into  the 
similarly  constituted  base  of  that  next  above  it,  are  conspicuous  not  only  in  the  structure 
of  Etna,  but  in  certain  sets  of  lavas  in  Madeira  and  in  the  island  of  Palma  in  the  Canaries, 
where  the  flows  of  melted  matter  seem  to  have  followed  each  other  at  intervals  of  time 
too  brief  to  allow  of  the  production  of  soils  by  decomposition,  and  where  consequently 
no  burnt  clays  of  a red  colour  mark  the  boundary  lines  between  successive  lavas.  If, 
however,  a great  interval  occurs  before  a fresh  current  takes  the  same  direction,  other 
causes  may  come  into  play  to  obliterate  the  superficial  roughness  of  the  exterior  of  an 
older  stream.  Thus,  for  example,  showers  of  comminuted  scorise  drifted  by  the  wind, 
spreading  usually  over  wider  areas  than  lavas,  may  fill  up  hollows ; floods  of  water- 
charged  with  sand  may  do  the  same ; the  action  of  the  sun,  rain,  frost,  and  vegetation 
causing  the  disintegration  of  rocks,  may  combine  to  make  the  successive  planes  of  stra- 
tification more  even  and  parallel  than  at  first,  though  it  is  true  that  in  most  of  these 
cases  the  red  or  burnt  tufis  would  betray  the  lines  of  separation. 
[Highly  inclined  stony  lava  of  modern  date  in  the  Cava  Secca  near  Zafarana. 
As  the  fact  of  lava  being  converted  into  a continuous  stony  mass  when  it  cools  on 
steep  slopes  will  perhaps  be  said,  even  if  admitted,  to  be  the  exception,  it  will  be 
desirable  to  multiply  examples  in  order  to  show  that  it  is  in  truth  the  rule.  By  refer- 
ence to  S.  VON  Walteeshausen’s  Map,  Plate  L.,  the  reader  will  see  the  position  of  a 
naiTow  valley,  about  300  feet  or  more  deep,  called  the  Cava  Secca,  about  a mile  W.N.W. 
of  Zafarana.  The  lavas,  scoriae,  and  tufis  intersected  by  this  ravine,  are  many  of  them 
among  the  oldest  exposed  to  view  on  Mount  Etna,  and  are  inclined  to  the  S.E.,  as  are 
those  in  the  neighbouring  valley  of  S.  Giacomo.  Near  the  lower  termination  of  the 
ravine,  at  the  point  a on  the  annexed  Map,  Plate  L.,  on  a steep  slope  above  the  right 
bank  of  the  torrent,  a comparatively  modern  lava  is  seen,  which  has  descended,  subse- 
quently to  the  excavation  of  the  whole  valley,  so  as  to  overflow  the  edges  of  the  older 
strata,  which  are  inclined  in  quite  a difierent  direction.  The  side  wall  of  this  modern 
current  has  been  undermined  and  partially  removed  by  aqueous  erosion,  just  as  it  has 
in  the  case  of  the  lava  of  1689  in  the  Cava  Grande  before  described,  p.  717.  The  section 
here  also  shows  a dip  of  35°  N.E.,  both  for  the  crust  of  scoriee  2-|  feet  thick,  and  for  the 
central  stony  layer  of  about  the  same  thickness.  The  lower  scoriae  are  also  exposed  to 
view  for  a depth  of  3 feet,  and  sometimes  more.  The  stony  stratum  is  somewhat  porous, 
yet  its  specific  gravity  is  no  less  than  2-554  when  first  plunged  into  water,  or  before  any 
air-bubbles  have  had  time  to  escape  from  its  inner  cells.  Many  crystals  of  felspar  and 
a small  quantity  of  ohvine  are  visible  in  the  rock.  I observed  this  modern  inclined  lava 
on  the  right  side  of  the  steep  mule-path  which  leads  up  from  the  Cava  Secca  to  a plat- 
