762 
SIE  CHAELES  LTELL  ON  THE  STEUCTHEE  OF  LAVAS 
5thly.  The  discontinuous  and  unconformable  arrangement  of  certain  parts  of  the 
ancient  and  modern  products  of  Etna  may  be  explained  by  supposing  the  former  exist- 
ence of  two  cones  as  above  stated,  and  by  the  truncation  of  the  ancient  summit  of  the 
mountain,  together  with  the  contemporaneous  or  subsequent  formation  of  the  Val  del 
Bove. 
6thly.  Although  the  cone-making  process  has  mainly  consisted  of  ordinary  eruptions, 
yet  the  present  steep  dip  of  some  of  the  old  lavas  and  beds  of  scoriae  has  been  modified, 
especially  near  the  ancient  foci  of  eruption,  by  subsequent  movements  accompanying  the 
rending  and  injection  of  the  rocks,  one-fifth  perhaps  of  the  present  inclination  of  the 
beds  being  due  to  this  cause,  instead  of  four-fifths,  as  required  by  the  elevation-crater 
hypothesis. 
7thly,  The  alleged  parallelism  and  uniform  thickness  of  the  beds  in  the  escarpments 
of  the  Val  del  Bove,  proves  on  closer  inspection  to  be  a delusion,  for  the  lavas  vaiy  in 
strength  and  frequently  thin  out,  being  only  persistent  for  great  distances  in  the  line  of 
their  dip,  or  the  direction  in  which  they  originally  flowed. 
Sthly.  The  ancient  and  highly  inclined  lavas  are  usually  free  fi’om  arches  and  flexm-es 
on  a great  scale,  bearing  a nearer  resemblance  to  those  parts  of  recent  cim-ents  which 
have  congealed  on  steep  slopes  than  to  those  which  have  cooled  on  more  level  ground. 
9thly.  The  scarcity  of  faults  in  the  lavas  of  various  ages,  and  the  fact  that  the  lavas  do 
not  cut  through  the  dikes  although  the  dikes  so  often  cut  through  them,  is  opposed  to 
the  doctrine  that  a considerable  amount  of  upheaval  has  been  due  to  the  injection  of 
lavas  in  conformable  sheets  between  pre-existing  beds  of  tutf  and  scoriae. 
lOthly.  As  the  dikes  are  of  various  ages  and  do  not  all  radiate  from  the  two  supposed 
centres  of  eruption,  the  verticality  of  so  many  of  them  is  inconsistent  uith  the  elevation- 
crater  hypothesis ; for  if  the  beds  were  originally  horizontal,  and  owed  theii’  present 
steep  dip  to  a final  catastrophe,  nearly  all  the  dikes  would  be  as  much  out  of  the  per- 
pendicular as  are  the  intersected  lavas  and  scoriae. 
Lastly.  The  absence  of  buried  lateral  cones  in  the  escarpments  of  the  Val  del  Bove. 
implies  that  the  earlier  eruptions  were  more  concentrated  and  more  limited  to  certain 
permanent  vents  than  the  modern  ones.]§3 
PART  III. 
ON  THE  EELATION  OF  THE  VOLCANIC  EOCKS  OF  MOHNT  ETNA  TO  THE  ASSO- 
CIATED ALLUVIAL  AND  MODEEN  TEETIAET  DEPOSITS,  WITH  CONCLUDING 
EEMAEKS  ON  CEATEES  OF  ELEVATION. 
Origin  of  the  Val  del  Bove,  and  how  far  due  to  aqueous  erosion. 
The  origin  of  that  large  crateriform  valley,  or  Caldera,  called  the  Val  del  Bove,  so 
often  alluded  to  in  the  foregoing  pages,  has  been  sometimes  attributed  to  a great  and 
sudden  catastrophe  connected  with  those  movements  which  are  supposed  to  have  given 
rise  to  the  mountain  itself,  and  to  have  caused  simultaneously  the  steep  outwai’d  dip  ot 
