758 
SIE  CHAELES  LTELL  ON  THE  STEECTTEE  OE  LATAS 
may  be  subdivided  into  several  sheets,  lying  one  over  the  other,  just  as  the  lava  of  1669 
is  seen  in  Catania,  in  the  shaft  artificially  cut  through  it  near  the  sea  in  the  Viha  Filip- 
pino, to  be  about  80  feet  thick,  but  separated  into  ten  beds,  most  of  them  with  scoria- 
ceous  partings,  caused  by  a succession  of  flows  during  the  same  eruption.  This  same 
lava  in  the  northern  suburbs  attains  a thickness  of  60  feet  in  one  magnificent  ciystalline 
mass,  without  any  such  bedding.  I allude  to  the  quarries  at  the  Botte  dell’Acqua, 
where  the  compact  stone  has  a light  grey  base  not  unhke  some  trachytes,  through  which 
are  scattered  well-formed  crystals  of  felspar  and  augite  with  some  olivine.  Let  us  now 
assume  the  narrowest  of  the  recent  lavas  above  mentioned  near  the  summit  of  Etna  to 
be  300  feet  broad,  and  suppose  one  of  the  subordinate  layers  to  be  8 feet  thick,  the 
analogy  of  all  the  sections  above  described  leaves  scarcely  any  doubt  that  it  would  con- 
sist of  an  upper  scoriaceous  crust,  say  3 feet  thick,  a middle  stony  layer  of  about  the 
same  thickness,  and  a bottom  scoriaceous  stratum  of  2 feet.  The  dip  just  below  the 
edge  of  the  platform  is  24°,  and  we  should  then  have  a highly  inchned  tabular  sheet  of 
stony  lava,  which,  in  reference  to  its  vertical  and  horizontal  dimensions,  would  be  in  the 
proportion  of  1 to  100,  forming  therefore  a thin  stratum.  We  have  ah’eady  considered 
(at  p.  735,  fig.  13)  what  would  be  the  appearance  of  several  such  juxtaposed  currents,  if 
seen  in  a transverse  section ; but  it  should  be  borne  in  mind  that  we  cannot  look  for  a 
strict  analogy  in  the  breadth,  average  thickness,  and  degree  of  compactness  in  lavas  of 
ancient  and  modern  cones,  unless  we  are  able  to  compare  sections  of  such  cones  at 
corresponding  heights,  which  is  usually  impossible. 
Flexures  and  arches  in  ancient  lavas. 
In  order  to  guard  against  imagining  a contrast  between  older  and  newer  lavas,  where 
none  really  exists,  we  must  remember  that  the  steeper  the  angle  at  which  a current 
has  cooled,  the  more  parallel  in  general  will  be  the  planes  of  stratification  of  the  several 
scoriaceous  and  stony  layers  which  compose  it.  No  doubt  a great  diversity  of  cha- 
racter prevails  in  this  respect  in  lavas  which  have  congealed  on  corresponding  dech- 
vities,  but  I never  met  with  ridges  and  furrows  of  great  dimensions  where  the  original 
slope  exceeded  an  angle  of  20° ; and  the  rare  occurrence  of  flexures  on  a gigantic  scale 
in  the  steeply  inclined  lavas  of  the  Val  del  Bove,  has  always  appeared  to  me  a fact  in 
favour  of  the  original  steep  dip  of  those  lavas.  We  have  ah’eady  seen  (p.  718,  fig.  6,  and 
p.  726,  fig.  9,  and  p.  730)  that  the  surfaces  of  the  lavas  of  1689  and  1852  are  compara- 
tively even  and  uniform  where  they  have  cooled  on  slopes  of  30°,  35°,  and  40° ; whereas 
at  points  immediately  contiguous,  where  the  same  currents  have  overflowed  less  inchned 
planes  of  10°  or  15°,  they  have  been  bent  into  gigantic  folds  and  ridges.  There  may  be 
exceptions  to  this  rule,  for  some  sharp  and  conspicuous  flexures  are  risible  in  the  escarp- 
ments of  the  Val  del  Bove,  as  for  example,  near  the  base  of  Monte  Zoccolaro,  in  which 
there  are  two  arches,  one  15  and  the  other  20  feet  high.  So  also  in  the  northern 
escarpment  above  Rocca  Capra  (see  Map  II.),  at  about  two-thirds  of  the  height  of  the 
entire  clifi"  above  its  base,  a cave  occurs  60  or  70  feet  high,  caused  by  the  arching  of 
