336 
EEV.  S.  HAUGHTOJs  OX  THE  PHTSICAE  STErCTUEE  OE  THE 
I shall  presently  describe  under  the  name  of  cleavage,  and  that  it  "was  the  first  system  of 
forces  that  acted  upon  the  coarse  shingle  of  the  Old  Eed  Sandstone  shore  after  its  depo- 
sition ; at  the  same  time  there  is  reason  to  think  that  the  conjugate  system  (A',  C')  was 
the  most  powerful  of  the  two  systems  of  forces.  Both  were  concerned  in  producing  the 
elevation  of  the  beds,  which  have  an  average  strike  of  E.  10°  46'  X.,  and  an  average 
dip  of  13°  42'  S.  The  strike  of  the  beds  is  thus  seen  to  be  intermediate  between  the 
systems  A and  A',  but  lying  nearest  the  former,  which  I conceive  to  have  acted  first  in 
order  of  time. 
Throughout  this  paper  I shall  speak  of  planes,  or  faults,  having  rectangular  bear- 
ings, as  forming  a conjugate  system ; assuming  the  well-known  mechanical  principles 
established  by  Cauchy,  who  has  shown  that  in  any  system  of  molecules  affected  with 
internal  tensions,  there  is  at  each  point  a system  of  three  orthogonal  planes,  corre- 
sponding with  the  three  principal  planes  of  the  ellipsoid  of  tension,  whose  centre  is 
situated  at  that  point.  These  orthogonal  planes  further  possess  the  property,  that  the 
internal  pressures  are  perpendicular  to  them,  and  that  one  of  them  is  the  plane  of  maxi- 
mum, and  another  the  plane  of  minimum  tension. 
3.  Joint  Planes. 
A joint  plane  is  a surface  of  division  in  a rock,  often  remarkably  smooth  and  clean 
cut,  without  the  slickensides  so  often  seen  in  faults,  rarely  accompanied  by  dislocation 
of  the  divided  beds,  and  generally  open  from  ^inch  to  3 or  4 inches.  A well-marked 
joint  plane  presents  so  striking  a resemblance  to  the  plane  surface  produced  by  crystal- 
lization, that  this  class  of  divisional  planes  in  rocks  has  always  been  confidently  appealed 
to  by  the  advocates  of  the  polar  or  crystalline  theory  of  rock  structure.  In  some  cases 
I have  no  doubt  that  this  resemblance  is  not  imaginary,  as  for  example  in  rock-masses 
of  igneous  origin,  such  as  granites  composed  of  uniform  proportions  of  quartz,  felspai'. 
and  mica,  or  hornblende.  Such  a rock-mass  is  truly  a definite  chemical  compound,  as 
much  so  as  any  mineral  on  a small  scale,  and  therefore  it  is  not  surprising  that  it  shoidd 
sometimes  present  joints  and  planes  intersecting  at  fixed  angles,  and  characterized  by 
all  the  properties  of  true  crystalline  faces.  Such  cases  have  come  under  my  ovn 
observation,  but  not  in  sufficient  numbers  to  entitle  me  to  state  any  definite  law, 
although  I think  much  might  be  accomplished  by  systematic  observations  directed 
specially  to  this  point.  However  this  may  be,  there  can  be  little  doubt  that  the  joints 
observed  in  a coarse  conglomerate  cannot  be  considered  due  to  crystallization,  whicli  can 
scarcely  be  supposed  to  have  acted  upon  a shingle  formed  of  chance  stones  Ijing  upon 
the  beach  of  the  Old  Red  Sandstone  seas. 
The  joint  surfaces  in  the  conglomerate  of  Waterford  are  beautifully  perfect,  generally 
open  1 to  4 inches,  nearly  vertical,  and  when  observed  in  the  sea  cliffs,  ginng  rise  to  an 
almost  columnar  structure,  in  consequence  of  the  sea  having  washed  out  huge  rectan- 
gular prisms  from  the  cliffs ; these  prismatic  caHties  and  prominences  being  invariably 
bounded  on  two  opposite  sides  by  joint  surfaces,  and  on  the  other  two  faces  by  the 
