OLD  EED  SANDSTONE  OE  THE  COUNTY  OE  WATEEEOED. 
337 
cleavage  surfaces  which  will  presently  be  described.  A fine  example  of  this  is  shown  in 
the  Conglomerate  Cliffs,  eastward  from  Portally  Head  towards  Oonarontia  and  Red 
Head : in  this  district  the  bedding  is  completely  masked  by  the  nearly  vertical  planes  of 
joint  and  cleavage  surfaces,  which  give  a rudely  prismatic  and  columnar  appearance  to 
the  cliffs.  Near  Swiny  Head  this  structure  is  remarkably  well  developed,  and  has  pro- 
duced a singular  effect : one  of  the  rude  columns  formed  by  the  joint  and  cleavage  surfaces 
has  fallen  partially  forward,  and  presents  an  appearance,  especially  when  seen  from  the 
sea,  which  resembles  some  of  the  old  stone  crosses  found  in  many  parts  of  Ireland. 
One  of  the  most  striking  chcumstances  connected  with  the  joint  surfaces  in  the  con- 
glomerate, is  the  fact  that  the  large  quartz  pebbles,  often  5 inches  diameter,  are  cut 
clean  across  as  if  with  a knife,  forming  a plane  rigorously  parallel  to  the  general  plane  of 
the  joint.  The  same  remarkable  fact  is  constantly  observed  in  the  joint  surfaces  of  the 
carboniferous  limestone  all  through  Ireland ; in  which  I have  frequently  observed  a fossil 
cut  in  two  by  the  joint  plane,  half  being  imbedded  in  each  portion  of  the  divided  rock, 
and  fitting  accui'ately  to  each  other  when  joined.  In  the  cleavage  planes  the  same 
circumstance  is  of  universal  occurrence  all  through  the  conglomerate  of  Waterford. 
According  to  my  idea  of  the  mechanical  origin  of  these  various  planes,  the  cleavage 
sui’faces  are  perpendicular  to  the  lines  of  maximum  force,  and  the  joint  surfaces  per- 
pendicular to  the  lines  of  minimum  force,  or  maximum  force  of  shrinkage,  if  I may  use 
a barbarous  word.  The  cleavage  structure  appears  to  have  been  first  developed  in  the 
rock,  and  the  jointed  structure  subsequently  added  by  the  shrinking  of  the  rock-mass, 
consequent  upon  evaporation  and  drying.  This  shrinking  and  contraction  of  the  mass 
would  naturally  show  itself  in  planes  perpendicular  to  the  lines  of  minimum  compres- 
sion. That  the  whole  rock-mass  was  pliant,  and  probably  even  soft,  at  the  time  the 
elevating  forces  were  applied,  appears  from  the  distortion  of  fossil  shells  in  shale  beds 
of  nearly  the  same  age  in  the  west  of  the  County  of  Waterford,  and  all  through  the 
south  of  the  County  of  Cork. 
At  fii’st  sight  it  might  appear  that  the  splitting  of  the  hard  quartz  pebbles  by  the  joint 
and  cleavage  planes  was  inconsistent  with  the  supposition  that  the  rock-mass  was  in  a 
yielding  condition  at  the  time  of  the  application  of  the  forces,  as  it  seems  to  indicate 
that  these  pebbles  were  held  fast,  as  in  a \ice,  by  the  hard  matrix  in  which  they  were 
imbedded  while  the  faces  that  split  them  were  in  action.  I think,  however,  that  inde- 
pendently of  the  conclusive  argument  derived  from  the  distortion  of  the  fossils,  it  is  not 
difficult  to  understand  how  a pliant  or  yielding  condition  of  the  mass  may  have  been  a 
necessary  condition  for  the  development  of  this  structure.  Let  us  suppose  the  clay, 
sand,  and  mud  of  some  landslip,  soft,  tough,  and  yielding,  with  its  imbedded  boulders 
and  pebbles  to  represent  the  Old  Red  Sandstone  conglomerate,  acted  on  by  external 
forces  of  compression,  and  by  its  own  weight ; at  each  point  of  the  mass  we  shall  have 
a plane  of  maximum  pressure,  and  one  of  minimum  pressure  at  right  angles  to  it ; and 
let  us  suppose  a plane  of  maximum  pressure  to  pass  through  several  boulders  or  pebbles, 
which  are  unyielding  substances  imbedded  in  a viscous  paste  which  presses  them  vari- 
2 z 2 
