WmCH  HAVE  CONSOLIDATED  ON  STEEP  SLOPES. 
777 
nation  probably  due  to  subsequent  movements,  and  if  so,  exactly  the  reverse  of  that 
which  the  theory  of  a central  upheaval  capable  of  lifting  up  the  cone  of  Mongibello 
would  require. 
(^\_Age  of  the  Marine  Tertiary  Strata  of  Cefali,  Catira  and  Nizzeti. 
In  order  to  determine  within  certain  hmits  the  geological  age  of  the  principal  mass 
of  Etna,  we  must  endeavour  to  ascertain  the  true  chronological  place  in  the  tertiary 
series  of  those  marine  strata  which  at  various  points  along  the  eastern  base  of  the 
mountain  crop  out  from  beneath  the  subamial  volcanic  rocks. 
I have  already  alluded  to  the  occurrence  of  such  fossiliferous  clays  in  the  district 
called  “Terra  Forte”  immediately  south  of  Catania,  where  they  must  have  emerged 
from  beneath  the  sea  at  a very  modern  period,  because  the  hills  composed  of  them  are 
capped  at  the  height  of  nearly  a thousand  feet,  not  only  by  unconformable  alluvium, 
but  by  formations  of  about  the  same  age  as  the  alluvium  containing  recent  species  of 
marine  shells.  We  again  meet  with  clays  hke  those  of  the  Terra  Forte,  replete  with 
fossil  shells  at  Cefali  in  the  northern  suburbs  of  Catania,  and  find  that  in  that  place 
also  they  are  covered  unconformably  by  alluvial  deposits  like  those  of  the  Simeto, 
proving  by  their  present  elevated  and  insulated  position  that  a considerable  change 
has  taken  place  in  the  physical  geography  of  the  country  since  they  originated.  At  a 
short  distance  north  of  Cefali,  we  come  to  the  inland  cliff  of  Fasano,  at  the  base  of 
which  we  again  observe  the  tertiary  clay,  while  in  the  cliff  itself  are  seen  the  overlying 
plant-bearing  tuffs  before  mentioned,  with  imbedded  pebbles,  like  those  of  the  alluvium 
of  the  valley  of  the  Simeto.  When  the  Fasano  chff  was  formed,  we  must  suppose  the 
sea  or  the  waters  of  the  ancient  estuary  to  have  beat  against  its  base,  causing  denudation, 
to  which  the  tuffs  could  never  have  been  exposed  after  they  had  once  attained  their 
present  level  (about  600  feet  above  the  sea),  and  when  the  surrounding  country  had 
assumed  its  present  configuration. 
The  most  inland  place,  within  the  boundary  of  the  volcanic  region  of  Etna,  where  the 
tertiary  strata  crop  out,  is  Catha,  about  two  English  miles  N.N.E.  of  Fasano  and  four 
north  of  Catania,  a point  at  which  they  also  reach  their  greatest  altitude,  ascertained 
by  S.  V.  Walteeshausen  to  be  1180  French,  or  1258  English  feet  above  the  sea. 
I visited  Catira  in  company  with  Signor  Geavina  and  Signor  Gaetano  G.  Gemmel- 
LAEO  in  1858.  The  last-mentioned  geologist  pointed  out  to  me  in  the  tertiary  clay  and 
sand,  several  pebbles  of  volcanic  rock  with  serpulse  attached,  implying  that  the  strata 
were  originally  of  littoral  origin,  and  that  some  volcanic  formation  not  only  pre-existed, 
but  had  emerged  in  the  neighbourhood  so  as  to  undergo  some  waste  and  supply  pebbles 
to  an  adjoining  beach. 
At  Catira  these  tertiary  strata  form  three  hills,  capped  with  doleritic  lava, — hills 
which  are  not  due  to  any  local  movements  of  upheaval,  but  to  the  action  of  the  sea, 
probably  at  the  time  when  they  were  gradually  emerging  from  the  deep.  Such  emi- 
nences may  once  have  formed  a group  of  islands  off  the  shore,  as  the  Fariglioni  or 
5 I 2 
