338 
EEV.  S.  HAUGHTON  ON  THE  PHTSICAE  STErCTUEE  OF  THE 
ously  in  different  dii’ections,  but  with  a decided  maximum  in  one  particular  direction. 
The  effect  of  this  pressure  will  be  to  cause  relative  motion  among  the  particles  of  the  paste, 
flattening  them  in  a direction  perpendicular  to  the  line  of  force,  and  so  causing  cleavage 
structure,  in  the  manner  so  admirably  illustrated  by  Mr.  Soebt’s  experiment  on  pipeclay 
and  scales  of  iron,  and  by  Mr.  Tyndall’s  experiment  on  wax.  In  the  meantime  the 
solid  pebbles  have  not  yielded  sensibly,  but  show  the  effect  of  the  enormous  pressure 
they  have  been  subjected  to,  by  developing  internally  a latent  cleaved  structure,  predis- 
posing them  to  split  most  readily  along  planes  perpendicular  to  the  line  of  maximum 
force.  In  this  manner  I would  account  for  the  sphttmg  of  the  quartz  pebbles  in  the 
planes  of  cleavage;  while,  in  the  case  of  the  joint  planes  which  were  formed  subse- 
quently, I conceive  that  the  rock  was  consolidated,  and  the  quartz  pebbles  held  fast  in 
their  now  hardened  paste,  while  the  shrinking  took  place  perpendicular  to  the  planes  of 
minimum  tension. 
This  mode  of  viewing  the  subject  is  confirmed  by  the  fact,  that  in  cleaved  limestone 
districts,  the  distorted  fossils  are  divided  by  the  joints  in  well-cut  planes,  showing  that 
the  rock  was  consolidated  when  the  joints  were  formed,  although  the  distortion  of  the 
fossil  proves  that  the  rock  was  soft  when  the  cleavage  took  place. 
4.  Cleamge  Planes. 
My  attention  was  originally  directed  to  the  study  of  the  physical  geology  of  the  red 
sandstone  conglomerate  of  Waterford  by  the  observation  of  two  circumstances;  the  fii’st 
of  these  was  the  rectangular  prismatic  appearance  of  the  conglomerate  chffs  already 
described;  and  the  second  was  the  detection  of  an  inner  structure  of  a peculiar  cha- 
racter, which  enabled  me  readily  to  distinguish  the  cleavage  from  the  joint  planes.  I 
found  that  the  coarse  conglomerate  had  in  many  places  acquu-ed  a platy  or  flaggy 
structure,  the  thickness  of  each  plate  varying  from  Jth  of  an  inch  to  4 inches,  the  fiat 
sides  of  the  plate  being  perfect  planes,  cutting  through  the  quartz  pebbles  m the  most 
beautiful  manner,  and  unaccompanied  by  any  dislocation  whatsoever.  The  moment 
I observed  this  peculiar  structure  in  the  conglomerate,  I felt  that  I had  foimd  the  key 
to  the  explanation  of  the  physical  structure  of  the  whole  district.  I had  before  noticed 
the  flaggy  appearance  assumed  by  the  cleaved  limestone  of  Cork  and  Kenmare,  and  tlie 
singular  platy  structure  of  many  of  the  quartz  rocks  and  grits  of  West  Galway,  and  had 
referred  them  to  incipient  cleavage,  rendered  imperfect  by  coarseness  of  texture  or  want 
of  viscosity ; the  possession  of  which  latter  quality  appears  to  be  the  principal  cause  of 
the  more  perfect  cleavage  observed  in  slates  and  mud  rocks  in  general ; but  I had  never 
thought  of  finding  cleavage  in  a coarse  conglomerate.  Having  discovered  the  appear- 
ance of  the  cleavage  proper  to  so  coarse  and  heterogeneous  a rock,  I instantly  deter- 
mined to  investigate  the  physical  structure  of  the  whole  district  with  reference  to  tliis 
question.  I determined  with  care  the  bearings  and  dips  of  345  joint  and  cleavage 
planes  along  the  coast,  from  Creadan  Head  to  Brownstown  Head  and  Binishark,  in- 
cluding about  thirty  observations  inland  made  in  quarries.  In  no  case  did  1 record  an 
