WHICH  HAVE  CONSOLIDATED  ON  STEEP  SLOPES. 
725 
in  many  of  the  modern  lavas  of  Etna,  and  such  empty  spaces  must  sometimes  at  a 
subsequent  period  be  unavoidably  filled  from  above  with  fused  matter,  which  may  then 
solidify  under  considerable  pressm’e,  giving  rise  to  masses  of  crystalline  rock,  and  offering 
a perplexing  problem  to  a geologist  if  he  should  obtain  a section  of  them  without  know- 
ing the  peculiar  conditions  under  which  they  originated. 
[My  friend  Mr.  Haetuxg,  during  his  late  exploration  of  the  Azores  (July  1857),  ob- 
seiwed  in  Pico,  one  of  the  middle  islands  of  that  group,  a subterranean  passage  of  great 
length,  traversing  lava  near  the  port  of  Cachorro,  on  the  N.N.W.  foot  of  the  volcanic  cone 
called  the  Pico,  the  highest  in  the  Azores.  The  lava  alluded  to  is  covered  with  vine- 
yards and  orchards,  and  supports  in  one  place  a village,  but  not  exactly  where  the  under- 
ground caverns  occur.  Mr.  Hartung  entered  this  natural  tunnel  for  several  hundred 
yai’ds,  and  found  in  the  interior  arched  caves,  some  20,  others  30  feet  high,  occa- 
sionally lowering  to  3 feet,  and  afterwards  enlarging  again.  In  one  place,  where  the 
thickness  of  the  roof  was  shghter  than  usual,  the  roots  of  some  fig-trees  were  seen 
penetrating  through  rents,  and  hanging  do’wn  into  the  cavern.  Such  subterranean 
cavities,  situated  only  170  feet  above  the  sea-level,  will  certainly,  says  Mr.  Hartung,  be 
some  day  flooded  by  a lava-ciu’rent  from  the  Peak,  in  which  case  the  vines  and  fruit- 
trees  may  escape  unharmed  in  spots  where  the  crust  of  scoriae  (so  bad  a conductor  of 
heat)  is  dense,  but  the  fig-trees,  whose  roots  pierce  into  the  caverns,  will  be  killed.] 
Aspect  of  the  recent  lava. — I shall  next  speak  of  the  bedding  and  internal  structure 
of  the  recent  lavas  above  described,  both  those  which  have  cooled  on  steep  slopes  and 
those  which  traverse  gently  inclined  ground.  It  was  in  the  northern  suburb  of  Zafarana 
that  we  first  encountered  the  great  southern  branch,  which  halted  there,  ending  in  a 
wall  30  feet  high,  and  inchned  at  an  angle  of  37°.  When  I saw  this  sterile  mass,  com- 
posed externally  of  fragmentary  materials,  and  listened  to  the  stories  told  me  of  the 
manner  in  which  it  had  crept  slowly  over  the  rich  pastures  and  vineyards,  utterly  destroy- 
ing them,  together  with  several  habitations,  I was  forcibly  reminded  of  the  like  devasta- 
tions and  losses  which  I had  witnessed  only  seven  weeks  before,  as  the  effects  of  a cause 
as  different  in  its  natime  as  can  well  be  conceived,  the  Alpine  glacier  of  the  valley  of 
Zermatt,  where  an  equally  steep  mound  of  rocky  fragments,  forming  the  frontal  moraine, 
had  been  pushed  forward  by  the  slow  but  irresistible  pressure  of  the  advancing  ice, 
over  green  meadows,  gardens,  and  chalets,  transforming  them  at  once  into  a wilderness, 
irreclaimable  for  ages. 
Passing  along  the  borders  of  the  lava,  we  soon  came  to  a point  where  numerous 
fumeroles  bore  testimony  to  the  heat  still  retained  in  the  interior.  The  white  vapours  had 
no  peculiar  odour,  seeming  to  consist  entirely  of  steam,  and  we  were  told  that  they  had 
been  unusually  copious  for  the  last  four  or  five  days.  My  companion.  Signor  Gaetano, 
suggested  that  the  heavy  rains  which  had  fallen  a week  before  might  be  the  cause,  the 
water  penetrating  to  the  deeper  and  hotter  parts  of  the  lava,  here  of  unusual  thickness, 
and  being  thus  converted  into  steam,  an  opinion  which  is  in  the  highest  degree  probable, 
'fhe  scoriaceous  surface,  kept  moist  in  some  places  by  the  warm  steam,  was  verdant 
