WHICH  ILiVE  CONSOLIDATED  ON  STEEP  SLOPES. 
763 
the  beds  sloping  away  on  all  sides  from  a central  axis.  But  if  the  section,  fig.  15,  p.  740, 
represent  with  any  approach  to  correctness  the  internal  structure  of  Etna,  and  if  the 
reasoning  advanced  in  the  foregoing  pages  be  sound,  the  reader  will  have  come  to  the 
conclusion,  that  the  whole  mountain,  with  its  lavas  and  tufis  sloping  away  from  more 
than  one  centre,  and  pierced  by  a succession  of  dikes,  was  already  complete  before  the 
Val  del  Bove  began  to  be  formed. 
In  the  first  edition  of  my  ‘ Principles  of  Geology,’  published  in  1830-33, 1 considered 
how  fax  this  great  valley  might  be  due  to  three  causes, — 1st,  engulfment ; 2ndly,  explo- 
sion ; and  3rdly,  aqueous  denudation ; and  I concluded  that  it  was  probably  due  for  the 
most  part  to  engulfment.  At  a later  period  (1849*)  I suggested  that  the  sea  might 
perhaps  have  once  entered  the  valley  and  exerted  a denuding  power,  an  opinion  which 
I did  not  maintain  in  later  editions  of  my  two  treatises  on  geology,  and  which  I entirely 
abandoned  after  visiting  Madeira  and  the  Canary  Islands  in  1854,  when  I became  aware 
of  the  prodigious  excavating  and  removing  power  which  torrents  and  rivers  can  exert  on 
a volcanic  mountain,  when  once  eruptions  have  ceased.  Mr.  Daxa  had  afready  called 
attention  to  this  fact  in  reference  to  certain  volcanos  in  the  Sandwich  Islands  f;  and 
M.  ZiEGLEE,  the  eminent  Swiss  geographer,  after  surveying  Madeira  and  publishing  a 
Map  of  the  island,  remarked  in  1856  to  Mr.  Haetung  and  me,  that  inclinations  such  as 
characterize  the  river-channels  of  that  island  would  be  excessive  and  quite  exceptional 
in  the  Alps. 
It  may,  indeed,  be  said  to  hold  true  generally  in  regard  to  volcanic  as  contrasted  with 
non-volcanic  mountains,  that  whereas  in  the  latter  the  valley-making  power  is  at  work 
during  the  whole  period  of  then’  growth,  that  same  agency  only  comes  into  play  in 
subaerial  volcanos  after  their  completion.  The  volume  of  rain-water  and  melted  snow 
absorbed  annually  by  a lofty  mountain  like  Etna  is  so  enormous,  that  if  at  length  the 
underground  di-ainage  is  converted  into  a superficial  one,  the  force  of  running  water 
operates  with  what  we  may  regard  as  a compensating  intensity,  as  if  the  waters  had  the 
power  of  making  up  for  lost  time. 
Alluvium  of  Giarre. — In  the  first  part  of  this  memoir  it  was  stated,  p.  709,  that  an 
allu'vial  formation  on  which  Giarre  and  other  towns  are  built,  and  which  sometimes  rises 
to  the  height  of  about  400  feet  above  the  level  of  the  sea,  attests  the  removal  at  some 
unknown  period  in  the  history  of  Etna,  of  a vast  quantity  of  stony  fragments  from  that 
part  of  the  mountain  which  lies  immediately  to  the  westward,  that  is  to  say,  in  the 
direction  of  the  Val  del  Bove.  If  it  could  be  shown  that  all  this  transported  matter 
came  down  from  the  great  valley  itself,  it  would  go  far  to  prove  that  the  abstraction  of 
the  missing  rocks  was  for  the  most  part  effected  by  aqueous  agency.  To  enable  the 
reader  to  judge  how  far  such  a conclusion  would  be  legitimate,  we  must  compare  the 
alluvial  formations  of  the  eastern  and  other  sides  of  Etna,  for  by  such  a comparison  it 
will  appear  that  those  at  the  eastern  base  of  the  mountain,  and  especially  those  opposite 
* Quarterly  Q-eological  Journal,  “ On  Craters  of  Denudation,”  &c.,  vol.  vi.  p.  207. 
t Geology  of  the  United  States  Exploring  Expedition,  1842. 
