Yol.  49.] 
THE  LLEYN,  AND  ASSOCIATED  VOLCANIC  ROCKS. 
151 
IV.  Fluidal  Structure  and  Flow-brecciation. 
Structure  which  is  probably  fluidal  can  be  traced  in  many,  more  or 
less  palagonitic,  green  rocks,  where  streaming  groups  and  streaky 
blotches  of  minute  epidote  or  augite-granules  are  drawn  up  to  wave¬ 
like  points.  Some  of  these  rocks  have  in  the  field  an  appearance  of 
flow-brecciation,  while  certain  microscopic  sections  prove  that  sub¬ 
sequent  movement  has  taken  place,  breaking  and  displacing  spheru- 
lites,1  so  that  fluxion  is  partly  masked  by  the  effects  of  mechanical 
deformation.  I  attribute  to  some  kind  of  fluidal  action  several  more 
obscure  examples  of  breccias  found  associated  with  volcanic  masses. 
Thus  a  dull  green  rock,  probably  a  devitrified  andesite  rather  acid  in 
composition,  encloses  compact  fragments  which  prove  to  be  more 
definitely  crystalline,  and  appear  to  have  been  carried  within  the 
surrounding  magma.'  Another  mass,3  in  the  field  resembling  an 
agglomerate,  when  examined  microscopically  seems  to  be  a  flow- 
breccia,  since  almost  all  the  fragments  are  alike,  and  are  cracked  and 
slightly  divided,  as  if  broken  by  the  effect  of  the  heated  magma  (now 
represented  by  palagonite  or  replaced  by  tufted  chlorite).  Possibly 
from  the  same  cause  a  separation  of  iron  oxide  and  of  a  clear  crys¬ 
talline  substance  has  taken  place  along  the  edges.  The  rock  is  a 
compact  porphyritic  diabase,  rarely  exhibiting  fluidal  structure  or  a 
few  amygdules,  and  including  one  or  two  pieces  of  a  brownish  glass. 
In  another  breccia,  with  fragments  of  a  reddish  slate,  sectors  of 
spherulites  are  embedded,  as  if  these  had  been  snapped  along  planes 
of  weak  cohesion  by  a  sudden  flow  of  lava  at  a  high  temperature. 3 
(PI.  I.  fig.  4.)  I  have  described  a  rock  from  Careg  in  which  purple 
spheroids  (4  to  5  inches  in  diameter)  seem  to  have  been  separated 
and  broken  by  a  second  influx  of  lava.5  Some  fragments  have  a 
roughly  concentric  fluidal  structure,  as  if  they  had  been  moved  while 
still  plastic,  and  towards  their  exterior  are  clearer  spots  bounded 
by  opacite,  which  possibly  are  incipient  spherulites.  (PI.  I.  fig.  3.) 
The  most  interesting  breccia,  however,  is  found  north  of  Porth 
Oer.  Elongated  spheroids  (9  to  15  inches  long)  of  a  dull-red 
ferruginous  basalt,  fractured  as  previously  described,  are  surrounded 
by  a  green,  schistose  matrix.  A  few  blocks  of  limestone  are  also 
included,  possibly  broken  from  a  small  mass  associated  with  the 
basalt.  This  has  been  affected  by  calcareous  infiltration  and  vein- 
ing  ;  while  the  limestone  has  acquired  a  colour  somewhat  similar 
to  that  of  t-he  igneous  rock  ;  but  it  can  be  distinguished  easily 
by  its  duller  texture  and  fracture,  its  paler  tint,  and  by  being 
broken  into  more  irregular  blocks.0  The  spheroids  of  basalt  appear 
1  In  a  slide  prepared  from  the  variolite,  Afon  Sant,  Aberdaron. 
2  This  rock  is  exposed  at  lowest  tide,  below  the  lime-kiln  north  of  Porth  Oer. 
3  Above  Pared-llech-y-menyn.  4  Prom  the  isthmus,  Dinas-fach. 
5  Geol.  Mag.  for  1892,  p.  412. 
6  This  breccia  appears  to  be  somewhat  similar  to  the  mass  at  Careg  Gwladys, 
Anglesey,  described  by  Mr.  J.  F.  Blake,  Quart.  Journ.  Geol.  Soc.  vol.  xliv.  (1888) 
pp.  510-11,  and  fig.  17  ;  but  at  Porth  Oer  the  inclusion  of  limestone-blocks  is  a 
very  subordinate  feature,  and  no  crystals  resembling  andalusite  occur.  The 
spheroidal  and  variolitic  structures  are  exhibited  in  more  than  one  mass  on  the 
beach.  One  of  these  shows  a  few  inclusions  of  limestone  on  its  north-eastern 
