756 
SIE  CHAELES  LTELL  ON  THE  STEIJCTrEE  OF  LAVAS 
among  other  irregularities,  three  beds  of  lava  (see  fig.  22),  yarying  from  4 to  6 feet  in 
thickness,  and  separated  by  strata  of  incoherent  matter.  T^Tien  followed  for  a distance 
Fig.  22. — Curvatures  in  the  lavas  of  Zoccolaro. 
of  between  200  and  250  feet,  the  middle  bed  thinned  out  at  c,  while  the  upper  and 
lower  lavas,  instead  of  being  about  40  feet  apart,  as  below  h,  bend  in  such  a manner  as 
to  come  within  12  or  14  feet  of  each  other,  as  at  c and  a. 
Upon  the  whole,  the  term  pseudo-parallelism,  which  Mr.  Haetuxg  and  1 were  in  the 
habit  of  applying  to  the  volcanic  beds  of  Madeira  and  the  Canaries,  seems  to  me  equally 
applicable  to  the  Etnean  formations,  when  these  are  analysed  in  detail. 
Analogous  form  and  arrangement  of  ancient  and  modern  lavas. 
It  may  still  be  asked,  are  there  not  single  continuous  beds  of  lava,  intersected  in  the 
escarpments  of  the  great  valley,  of  wider  extension  than  we  can  possibly  explain  on  the 
hypothesis  of  then’  having  flowed  down  the  flanks  of  a volcanic  cone  in  the  manner  of 
ordinary  lavas  1 
Before  we  can  reply  to  this  question,  we  have  to  consider  two  points ; namely,  first, 
whether  the  ancient  rocks  to  which  we  refer  are  intersected  longitudinally,  or  in  a direc- 
tion transverse  to  their  dip ; and  secondly,  if  transversely,  what  accui’ate  data  have  we 
respecting  the  average  width  of  such  beds,  whether  among  the  older  lavas  of  Etna,  as 
seen  in  the  V al  del  Bove ; or  in  the  modern  products  of  Etna,  V esuUus,  or  any  known 
volcano  1 As  to  the  first  question,  it  is  clear  that  if  our  sections  are  longitudinal,  that  is, 
in  the  direction  in  which  the  lavas  flowed,  there  is  no  reason  why  a stony  layer  should 
not  be  continuous  for  several  miles.  Even  if  the  section  be  diagonal  in  reference  to  the 
original  course  of  the  stream,  a single  layer  may  be  continuous  for  as  great  a distance  as 
we  are  likely  to  have  an  opportunity  of  tracing  it  in  the  Val  del  Bove.  The  prevailing 
dip  towards  the  east  of  the  numerous  beds  there  seen,  both  m the  northern  and 
southern  escarpments,  accompanied  as  it  is  by  a diminishmg  height  of  the  moimtain 
as  it  stretches  eastward,  implies,  according  to  the  theory  of  eruption,  that  the  currents 
of  melted  matter  flowed  eastward,  and  are  therefore  cut  open  longitudinally  or  more  or 
less  diagonally ; whereas  the  lavas,  if  any,  which  came  do'nm  from  the  axis  of  Mongi- 
bello  after  the  overwhelming  of  the  cone  of  Trifoglietto,  would  naturally  take  the  same, 
or  nearly  the  same  direction. 
cs[In  regard  to  the  second  question,  if  we  admit  that  some  of  the  lavas  and  agglome- 
rates which  depend  on  the  axis  of  Trifoglietto  are  really  intersected  in  a direction  at 
