WHICH  HAVE  CONSOLIDATED  ON  STEEP  SLOPES. 
755 
fault  shifting  the  beds  vertically  for  several  yards ; but  we  soon  satisfied  ourselves  that  a 
gradual  change  in  the  thickness  of  particular  beds  afforded  the  true  explanation.  The 
ravine  alluded  to  is  in  the  Serra  del  Solfizio,  about  midway  between  the  Rocca  del  Corvo 
and  the  hill  of  Zoccolaro  (see  Map,  Plate  L.).  It  is  about  300  feet  wide,  and  runs  north 
and  south.  The  strata  consist  chiefly  of  agglomerates,  some  containing  much  scoriae 
and  large  angular  pieces  of  lava.  We  may  expect  such  beds  to  be  more  continuous  and 
uniform  in  thickness  throughout  wide  spaces  on  the  flanks  of  a volcano  than  single 
currents  of  lava,  because  dmlng  violent  eruptions  the  ejectamenta  are  scattered  by  the 
explosive  action  of  gases  and  by  the  winds  over  extremely  large  areas.  The  first  mass, 
about  80  feet  thick,  which  we  examined  near  the  base  of  the  cliff,  on  the  west  side  of  the 
ravine,  was  di-vided  into  beds  which  varied  in  their  dip  from  24°  to  28°,  when  viewed  in 
a north  and  south  direction ; while  in  the  east  and  west  section,  or  that  which  faces  the 
Val  del  Bove,  we  found  a want  of  parallelism  in  the  same  beds  amounting  to  15°  in  the 
space  of  a few  hundred  yards.  In  the  above-mentioned  80  feet  there  were  only  six 
layers  of  lava,  the  united  thickness  of  which  was  no  more  than  10  feet.  We  next 
observed  another  vertical  mass,  on  the  east  side  of  the  same  ravine,  where  the  agglome- 
rates were  in  like  manner  very  preponderant  throughout  a thickness  of  300  feet,  the  dips 
varying  from  28°  to  18°,  and  diminishing  as  we  ascended.  Immediately  above  we  came 
to  another  section,  represented  in  the  annexed  diagram  (fig.  21).  At  the  bottom  are 
Eig.  21, — PFant  of  parallelism  in  the  heds  in  the  Berra  del  Solfizio,  south  side  of  the  Val  del  JBove. 
several  beds  of  agglomerate  [a  to  Z>);  then  a doleritic  lava  (c),  about  3 feet  in  its  greatest 
thickness,  and  inclined  at  27°  S. ; over  that  again  a bed  of  scoriae  and  agglomerate  [d], 
5 feet  thick,  which  at  the  distance  of  a few  yards  thinned  out  entirely  at  g ; above  this 
is  another  bed  of  scoiiae  and  agglomerate  (e),  5 feet  thick ; and  over  that  a lava  [f], 
12  feet  thick,  inclined  at  10°;  so  that  here,  in  a vertical  height  of  little  more  than 
10  feet,  the  want  of  parallelism  of  the  two  lavas  c and  amounted  to  17°. 
At  the  eastern  extremity  of  the  Serra  del  Solfizio  in  the  hill  of  Zoccolaro,  we  observed, 
