WHICH  BATE  CONSOLIDATED  ON  STEEP  SLOPES. 
759 
the  lava  and  the  washing  out  of  the  lower  seorise.  But  these  and  other  caves  are 
exceptional,  and  the  circumstances  under  which  they  occur  deserve  particular  investiga- 
tion, as  they  may  perhaps  be  due  to  some  local  peculiarity  in  the  external  form  of  the 
ancient  cone  at  such  points.]83 
Bikes  in  the  Val  del  Bove. 
Dr.  Caelo  Gemmellaeo,  in  several  of  his  memou’s  on  Mount  Etna  published  in  the 
years  1835, 1847,  and  1854*,  &c.,  has  argued  against  the  “ Elevation-crater”  hypothesis, 
by  calling  attention  to  the  steepness  of  the  slopes  down  which  some  of  the  modern 
lavas  of  Etna  have  flowed ; and  he  has  particularly  insisted  on  a fact,  first  pointed  out  by 
his  brother  Maeio  Gemmellaeo,  that  a great  number  of  dikes  radiate  from  the  present 
centre  of  Mongibello.  Baron  S.  vox  Walteeshausex,  during  his  patient  explorations 
of  Etna,  has  also  observed  (see  above,  p.  741),  that  many  dikes,  thirteen  or  more  in 
number,  composed  of  greenstone,  converge  in  hke  manner  towards  an  ancient  centre, 
called  by  us  the  axis  of  Trifoglietto.  Assuming  that  all  these  dikes  have  been  originally 
vertical,  or  nearly  so,  they  might  retain  their  verticality  even  after  upheaval,  if  they 
radiated  in  exactly  the  same  directions  as  the  movements  which  uplifted  the  strata  so  as 
to  make  them  dip  away  quaquaversally  from  a central  axis.  But  in  that  case  we  should 
be  under  the  necessity  of  concluding  that  the  upheaval  of  the  principal  centre,  namely 
that  of  Mongibello,  has  not  interfered  with  or  disturbed  the  position  of  the  beds 
dependent  on  the  lesser  and  adjoining  cone  of  Trifoglietto,  a conclusion  quite  incon- 
ceivable. All  observers  agree  that  there  are  a multitude  of  dikes  which  do  not  radiate 
from  either  of  the  two  principal  centres  above  alluded  to,  and  it  is  impossible  to 
reconcile  the  prevailing  perpendicularity  of  the  dikes  with  the  steep  inclination  of  the 
lavas  and  fragmentary  strata  which  these  same  dikes  intersect,  unless  we  abandon  the 
upheaval  theory.  For  if  a set  of  horizontal  strata  traversed  by  vertical  dikes,  radiating 
from  more  than  one  centre,  and  by  other  dikes  not  converging  to  any  single  point,  were 
uplifted  so  as  to  dip  at  angles  of  20°  and  30°,  it  is  obvious  that  the  dikes  also  must 
become  as  much  inclined  relatively  to  the  horizon  as  are  the  strata,  the  only  difference 
being  that  the  dip  of  the  dikes  and  of  the  strata  would  be  in  opposite  directions  f. 
Of  the  fljst  three  dikes  which  I measured  in  the  hill  of  Calanna,  I found  two  which 
were  vertical,  the  direction  of  one  being  south-west  and  of  the  other  30°  east,  while  the 
third,  which  had  also  a south-east  direction  (on  the  side  nearest  to  Zoccolaro),  was  no  less 
than  30°  out  of  the  perpendicular,  its  dip  or  hade  being  60°  south-west.  In  the  Serra  del 
Solflzio  and  elsewhere  I saw  many  dikes  which  were  vertical,  where  the  beds  intersected 
by  them  were  highly  inclined,  while  other  dikes  which  were  out  of  the  perpendicular, 
were  not  inclined  in  the  direction  which  the  upheaval  theory  would  require,  but  often 
precisely  the  reverse,  their  hade  being  towards  the  same  point  of  the  compass  as  the 
* SuUa  Costituzione  fisica  deU’  Etna,  1847  ; and  others  in.  the  ‘ Atti  deUa  Acad.  Gioenia.’ 
t See  my  statement  of  this  argument,  with  a diagram,  in  the  6th  vol.  of  the  Geol.  Quart.  Journal,  p.  281, 
1850;  and  Principles  of  Geology,  9th  edit.  p.  418. 
MDCCCLVIII.  5 G 
