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MISS  C.  A.  RAISIN'  ON  VARIOLITE  OE 
[May  1893, 
here,  as  at  other  places,  to  become  green  and  palagonitic  at  the  exte¬ 
rior,  especially  where  segregation  of  the  iron  oxide  marks  a  tendency 
towards  the  formation  of  spherulites.  But  several  adjacent  rocks, 
although  like  this  mass  affected  by  subsequent  crushing,  seem  to  be 
examples  of  flow  or  flow-brecciation.  A  separation  in  the  magma 
was  probably  followed  by  a  second  movement  of  lava.  Although  this 
did  not  violently  displace  or  carry  along  the  spheroids  of  basalt,  it 
seems  possible  that  it  partially  thrust  itself  between  them.  If  not, 
we  have  first  to  account  for  the  difference  in  microscopic  structure 
and  in  fracture  as  being  due  to  the  glassy  and  the  cryptocrystalline 
characters  of  the  outer  and  inner  parts  of  the  spheroid  respectively, 
and  to  the  different  results  of  crushing  upon  each.  But,  secondly, 
the  analyses  given  previously  suggest  that  the  two  rocks  might  be 
found  to  have  a  marked  difference  in  chemical  composition.  On 
this  point  further  investigation  would  be  necessary. 
In  certain  neighbouring  masses,  which  are  probably  flow-breccias, 
the  fragments  are  more  separated,  irregular,  and  smaller,  but  some 
are  rounded,  possibly  from  being  moved  while  still  plastic.  These 
indicate  a  gradation  towards  a  variolite,  although  they  have  not  a 
radial  structure.* 1 
Y.  Yariolite. 
(a)  General  Microscopic  Structure. — Corresponding  to  the  two 
types  of  rock  are  two  forms  of  variolite.2  One  is  a  compact  green 
diabase  exhibiting  light-coloured  spots,  which  on  microscopic  exa¬ 
mination  prove  to  be  spherulites,  generally  elliptical,  greyish,  and 
duller  than  the  surrounding  matrix.  They  include  a  felspathic 
constituent  showing  radial  aggregation  in  polarized  light ;  also 
minute,  brownish,  brightly  polarizing  granules,  probably  augite; 
and  a  fine  dust,  which  is  much  less  abundant  in  the  central  and  con¬ 
sequently  clearer  part.  Towards  the  periphery  a  zone  of  pseudo- 
crystallites  occurs.  The  brecciated  spherulites  at  Diuas-fach  are 
similar,  but  are  much  clearer.  In  the  second  type  (PI.  I.  fig.  5) 
needles  of  brownish  iron  oxide  are  grouped  in  feathery  tufts  or 
elongated  sheaves.  The  radiate  character  is  found  only  at  the 
centre  of  the  spherulite,  or  both  there  and  at  the  periphery,  or  it  is 
face  or  edge,  -while  the  larger  surface  exposed  to  the  north-west  might  be  roughly 
represented  by  the  figure  given  by  Mr.  Blake,  if  the  ‘  fragments  ’  were  less 
angular,  more  uniformly  fractured,  more  irregularly  disseminated,  and  consisted 
entirely  of  dull  red  basalt.  From  the  details  given  in  the  description  of  the 
Careg  Glwladys  variolite  by  Prof.  Cole,  Sci.  Proc.  Boy.  Dub.  Soc.  vol.  vii.  (1891) 
p.  112,  I  gather  that  the  calcareous  inclusions  are  mainly  modified  igneous 
rock,  and  that  the  mass  is  therefore  probably  very  similar  to  this  from  Porth 
Oer.  Owing  to  bad  weather  during  my  visits  to  the  Anglesey  district,  I  did  not 
reach  the  particular  boss  figured  by  Mr.  Blake,  but  a  general  lithological  simi¬ 
larity  was  evident. 
1  I  may  here  refer  to  my  description  in  a  former  paper  of  structures  con¬ 
nected  with  pyromerides  in  acid  rocks,  Quart.  Journ.  Geol.  Soc.  vol.  xlv.  (1889) 
pp.  255,  267  (2nd  group). 
2  The  first  type  is  illustrated  in  rocks  from  near  Aberdaron  by  Afon  Sant  and 
at  Deunant ;  the  second  from  Porth  Orion,  Dinas-fach,  and  Porth  Oer. 
