752 
SIK  CHAELES  LTELL  ON  THE  STErCTTEE  OF  LAVAS 
origin  to  most  of  those  lavas  of  Sciuricosino  near  Zafarana,  vrhich  are  bent  into  arches, 
and  which  thin  out  in  both  dhections,  as  represented  by  him  in  plate  20  of  his  ‘ Atlas.’ 
Yet  these  occur  far  from  the  region  where  dikes  are  frequent,  and  where  we  have  posi- 
tive proof  of  the  injection  of  melted  matter  into  rents.  That  near  the  great  centres  of 
eruption,  where  the  hade  of  the  many  dikes  is  so  much  out  of  the  perpendicular,  and  where 
some  are  tortuous,  and  are  seen  to  cross  and  shift  others,  there  should  he  found  occasion- 
ally sheets  of  intrusive  matter  parallel  to  the  tuffs  and  older  lavas,  will  be  readily  con- 
ceded. Mr.  Haetung  and  I observed  in  Madeira,  at  the  western  extremity  of  Cape 
Giram,  some  dikes  which  were  nearly  horizontal,  and  had  been  injected,  for  a certain 
portion  of  their  course,  between  sheets  of  pre-existing  lava ; but  these  dikes,  like  some 
of  similar  origin  above  Guimar  in  the  Island  of  Teneriffe,  were  ascertained  by  us,  when 
we  traced  them  for  30  or  40  feet,  to  cut  through  the  regular  lavas  and  tuffs ; and  in 
the  case  of  Cape  Giram  in  Madeira,  they  give  rise  to  faults  in  the  older  beds,  a rare 
phenomenon  in  Madeira,  as  it  is  also  on  Mount  Etna. 
Had  the  lavas  which  slope  away  from  the  ancient  centres  of  Trifoghetto  and  Mongi- 
bello  been  in  great  part  injected  between  the  tuffs,  we  should  have  frequently  seen 
them  penetrating  through  the  dikes.  But  though  these  last  are  of  so  many  different 
ages,  and  are  continually  seen  to  traverse  the  alternating  lavas  and  tufis,  I could  discover 
no  instance  of  such  dikes  being  in  their  turn  traversed  by  the  lavas.  It  may  be  asked 
how,  in  the  escarpments  of  the  Val  del  Bove,  we  can  distinguish  a lava,  which  has  flowed 
originally  at  the  surface,  from  a tabular  mass  of  rock,  which  may  have  been  forced,  when 
in  a melted  state,  into  a fissure  betwen  two  layers  of  tuffl  I reply,  that  the  lava  has 
almost  invariably  its  upper  and  lower  scoriae,  and  sometimes  immediately  beneath  the 
latter  a red  layer  of  burnt  tuff,  such  as  I saw  in  the  Balzo  di  Trifoglietto,  at  various 
heights,  and  in  Monte  Zoccolaro  and  in  the  valley  of  S.  Giacomo,  where  I traced  a red 
tuff  for  a great  distance  underlying  the  most  powerful  of  the  older  lavas.  Such  red 
layers  are  never  in  direct  contact  with  the  central  and  overljing  crystalline  stonv  layer, 
for  there  intervenes  always  a fundamental  stratum  of  fragmentary  or  scoriaceous  matter 
between  the  stony  bed  and  the  burnt  tuff  below.  On  the  other  hand,  I looked  hi  vam 
for  an  instance  of  some  powerful  sheet  of  lava  which  had  one  of  these  brick-red  clays 
above  as  well  as  below  it.  Had  the  crystalline  lavas,  whether  trachytic  or  doleritic. 
whether  slightly  or  steeply  inclined,  been  in  great  part  intrusive,  they  would  have 
altered  the  tuffs  as  much  above  as  below  them.  Moreover,  they  must  have  given  rise 
to  innumerable  faults;  for  while  they  vary  in  thickness  from  3 to  60  feet,  they  are 
not,  as  I shall  show  in  the  sequel,  persistent  for  indefinite  distances,  but  often  thin  out 
rapidly  in  both  directions.  They  ought,  therefore,  had  they  been  injected,  to  have  lifted 
up  the  incumbent  deposits  partially,  so  as  to  give  rise  to  many  conspicuous  faults.  For 
these  reasons,  I cannot  adopt  the  conclusion  that  the  uplieaval  of  Etna  has  been  lai'geiy 
due  to  the  injection  of  lavas  in  sheets  parallel  or  conformable  to  the  tuffs  and  frag- 
mentary materials.] 
